try to make less row fetches by searching for rows on condition period_start < (period_end of new_data)

* The overlaps check is implemented on a handler level per row command.
  It creates a separate cursor (actually, another handler instance) and
  caches it inside the original handler, when ha_update_row or
  ha_insert_row is issued. Cursor closes on unlocking the handler.

* Containing the same key in index means unique constraint violation
  even in usual terms. So we fetch left and right neighbours and check
  that they have same key prefix, excluding from the key only the period part.
  If it doesnt match, then there's no such neighbour, and the check passes.
  Otherwise, we check if this neighbour intersects with the considered key.

* The check does introduce new error and fails with ER_DUPP_KEY error.
  This might break REPLACE workflow and should be fixed separately

recv_sys_t opened redo log files along with log_sys_t. That's why I
removed file sharing logic from InnoDB
in 9ef2d29f
But it was actually used to ensure that only one MariaDB instance
will touch the same InnoDB files.

os0file.cc: revert some changes done previously

mapped_file_t::map(): now has arguments read_only, nvme

file_io::open(): now has argument read_only

class file_os_io: make final

log_file_t::open(): now has argument read_only

We introduce an EXTENDED log record for appending an undo log record
to an undo log page. This is equivalent to the MLOG_UNDO_INSERT record
that was removed in commit f802c989,
only using more compact encoding.

mtr_t::log_write(): Fix a bug that affects longer log
record writes in the !same_page && !have_offset case.
Similar code is already implemented for the have_offset code path.
The bug was unobservable before we started to write longer
EXTENDED records. All !have_offset records (FREE_PAGE, INIT_PAGE,
EXTENDED) that were written so far are short, and we never write
RESERVED or OPTION records.

mtr_t::undo_append(): Write an UNDO_APPEND record.

log_phys_t::undo_append(): Apply an UNDO_APPEND record.

trx_undo_page_set_next_prev_and_add(),
trx_undo_page_report_modify(),
trx_undo_page_report_rename():
Invoke mtr_t::undo_append() instead of emitting WRITE records.

We introduce an EXTENDED log record for initializing an undo log page.
The size of the record will be 2 bytes plus the optional page identifier.
The entire undo page will be initialized, except the space that is
already reserved for TRX_UNDO_SEG_HDR in trx_undo_seg_create().

mtr_t::undo_create(): Write the UNDO_INIT record.

trx_undo_page_init(): Initialize the undo page corresponding to the
UNDO_INIT record. Unlike the former MLOG_UNDO_INIT record, we will
initialize almost the entire page, including initializing the
TRX_UNDO_PAGE_NODE to an empty list node, so that the subsequent call
to flst_init() will avoid writing log for the undo page.

Now there can be only one log file instead of several which
logically work as a single file.

Possible names of redo log files: ib_logfile0,
ib_logfile101 (for just created one)

innodb_log_fiels_in_group: value of this variable is not used
by InnoDB. Possible values are still 1..100, to not break upgrade

LOG_FILE_NAME: add constant of value "ib_logfile0"
LOG_FILE_NAME_PREFIX: add constant of value "ib_logfile"

get_log_file_path(): convenience function that returns full
path of a redo log file

SRV_N_LOG_FILES_MAX: removed

srv_n_log_files: we can't remove this for compatibility reasons,
but now server doesn't use this variable

log_sys_t::file::fd: now just one, not std::vector

log_sys_t::log_capacity: removed word 'group'

find_and_check_log_file(): part of logic from huge srv_start()
moved here

recv_sys_t::files: file descriptors of redo log files.
There can be several of those in case we're upgrading
from older MariaDB version.

recv_sys_t::remove_extra_log_files: whether to remove
ib_logfile{1,2,3...} after successfull upgrade.

recv_sys_t::read(): open if needed and read from one
of several log files

recv_sys_t::files_size(): open if needed and return files count

redo_file_sizes_are_correct(): check that redo log files
sizes are equal. Just to log an error for a user.
Corresponding check was moved from srv0start.cc

namespace deprecated: put all deprecated variables here to
prevent usage of it by us, developers

We plan use the redo log record main type code 0x20 for
InnoDB specific index page operations.

mrec_type_t: Rename INIT_INDEX_PAGE to EXTENDED.

mrec_ext_t: The EXTENDED subtypes.

This is a non-functional change: the redo log record encoding
that was introduced in commit 7ae21b18
is not affected.

btr_page_reorganize_low(): Log only the changed data in the page.

TODO: Do not copy the entire changed payload to the redo log.
Emit a combination of MEMMOVE and WRITE records to reduce the log volume.

commit 08ba3887 of MDEV-12353
introduced an incorrect assumption, which was documented by
the failing assertion.

After instant ADD COLUMN, we can have a null (and in-place) UPDATE
of NULL to NULL. No data needs to be written for such updates.

For ROW_FORMAT=REDUNDANT, we reserve space for the NULL values,
and to be compatible with existing behaviour, we will zerofill
the unused data bytes when updating to NULL value.

trx_purge_free_segment(): In some cases (observed when running
the test innodb_zip.wl5522_debug_zip), there is no change to
the TRX_UNDO_NEEDS_PURGE field. Add mtr_t::OPT to disable a debug check.

The bogus debug check was introduced in
commit 56f6dab1.

We add FIXME comments and some sketch code for the following cases:

It is possible to write considerably less log for ROW_FORMAT=COMPRESSED
pages. For now, we will delete the records one by one.

It is also possible to treat 'deleting the last records' as a special
case that would involve shrinking PAGE_HEAP_TOP. That should reduce
the need of reorganizing pages.

page_mem_free(): When deleting the very last record of the page,
even if the record did not fully utilize all bytes in a
former PAGE_FREE record, truncate the PAGE_HEAP_TOP and reduce
PAGE_GARBAGE by the saved amount.

fsp_page_create(): Always initialize the page. The logic to
avoid initialization was made redundant and should have been removed
in mysql/mysql-server@ce0a1e85e24e48b8171f767b44330da635a6ea0a
(MySQL 5.7.5).

btr_store_big_rec_extern_fields(): Remove the redundant initialization
of FIL_PAGE_PREV and FIL_PAGE_NEXT. An INIT_PAGE record will have
been written already. Only write the ROW_FORMAT=COMPRESSED page payload
from FIL_PAGE_DATA onwards. We were unnecessarily writing from
FIL_PAGE_TYPE onwards, which caused an assertion failure on recovery:

	recv_sys_t::alloc(size_t): Assertion 'len <= srv_page_size' failed

when running the following tests:

	./mtr --no-reorder innodb_zip.blob,4k innodb_zip.bug56680,4k