If online alter fails, TABLE_SHARE can be freed while concurrent
transactions still have row events in their online_alter_cache_data.
On commit they try'll to flush them, writing to TABLE_SHARE's
Cache_flip_event_log, which is already freed.
This causes a crash in main.alter_table_online_debug test

don't simply set tdc->flushed, use flush_unused(1) that removes opened
but unused TABLE instances (that would otherwise prevent TABLE_SHARE from
being closed by keeping the ref_count>0).

even if called from binlog_rollback()

because some engines (e.g. rocksdb) only do it readonly.
instead, downgrade the lock after reading the first row.

instead use only one (trx) IO_CACHE and truncate it if the
statement is rolled back.

don't use binlog_cache_mngr to accumulate the data,
use binlog_cache_data instead.

(binlog_cache_data owns one IO_CACHE, binlog_cache_mngr owns
two binlog_cache_data's, trx and stmt).

because ALTER TABLE ... DROP SYSTEM VERSIONING
is not just a change in the table structure, it also deletes
all historical rows

also

* don't modify write_set
* backup/restore rpl_write_set

so any other value of ALGORITHM or LOCK disables online alter

use ht->start_consistent_snapshot() instead

no functional changes here

two new tests:
* alter table times out because of a long concurrent trx
* alter table adds a column in the middle

* Log rows in online_alter_binlog.
* Table online data is replicated within dedicated binlog file
* Cached data is written on commit.
* Versioning is fully supported.
* Works both wit and without binlog enabled.

* For now savepoints setup is forbidden while ONLINE ALTER goes on.
  Extra support is required. We can simply log the SAVEPOINT query events
  and replicate them together with row events. But it's not implemented
  for now.

* Cache flipping:

  We want to care for the possible bottleneck in the online alter binlog
  reading/writing in advance.

  IO_CACHE does not provide anything better that sequential access,
  besides, only a single write is mutex-protected, which is not suitable,
  since we should write a transaction atomically.

  To solve this, a special layer on top Event_log is implemented.
  There are two IO_CACHE files underneath: one for reading, and one for
  writing.

  Once the read cache is empty, an exclusive lock is acquired (we can wait
  for a currently active transaction finish writing), and flip() is emitted,
  i.e. the write cache is reopened for read, and the read cache is emptied,
  and reopened for writing.

  This reminds a buffer flip that happens in accelerated graphics
  (DirectX/OpenGL/etc).

  Cache_flip_event_log is considered non-blocking for a single reader and a
  single writer in this sense, with the only lock held by reader during flip.

  An alternative approach by implementing a fair concurrent circular buffer
  is described in MDEV-24676.

* Cache managers:
  We have two cache sinks: statement and transactional.
  It is important that the changes are first cached per-statement and
  per-transaction.
  If a statement fails, then only statement data is rolled back. The
  transaction moves along, however.

  Turns out, there's no guarantee that TABLE well persist in
  thd->open_tables to the transaction commit moment.
  If an error occurs, tables from statement are purged.
  Therefore, we can't store te caches in TABLE. Ideally, it should be
  handlerton, but we cut the corner and store it in THD in a list.

Event_log is supposed to be a basic logging class that can write events in
a single file.

MYSQL_BIN_LOG in comparison will have:
* rotation support
* index files
* purging
* gtid and transactional information handling.
* is dedicated for a general-purpose binlog

* Eliminate most usages of THD::use_trans_table. Only 3 left, and they are
  at quite high levels, and really essential.
* Eliminate is_transactional argument when possible. Lots of places are
  left though, because of some WSREP error handling in
  MYSQL_BIN_LOG::set_write_error.
* Remove junk binlog functions from THD
* binlog_prepare_pending_rows_event is moved to log.cc inside MYSQL_BIN_LOG
  and is not anymore template. Instead it accepls event factory with a type
  code, and a callback to a constructing function in it.

pump up binlog and cache manager to level of binlog_log_row_internal

1. Change m_size to uint. This removes some implicit conversions.
  See unpack_row, for instance:
  uint max_cols= MY_MIN(tabledef->size(), cols->n_bits);
2. Improve table_def memory layout by reordering columns

alter_table_online -> alter_table_locknone
gis-alter_table_online -> gis-alter_table