dict_index_t::instant_metadata(): For instantly dropped NOT NULL
variable-length columns, store an empty string for the default value.

This reverts commit 2d4075e1
where the debug assertion was added. There seems to be a potential
problem in the purge of indexes that depend on virtual columns.

Ultimately, we should change the InnoDB undo log format so that
all actual secondary index keys are stored there, also for
virtual or spatial indexes. In that way, purge and rollback would
be more straightforward.

This works around a crash in purge.

On Windows, a mismatch between TIMETPF ("%ld") and time_t would be reported.
Use "%ld" and long, like the code used to be.

It seems mysqltest does not print warnings raised at mysql_prepare() time.

Suppressing such --ps dependent warnings.

Additional changes:

1. Adding a fast path for ASCII characters
2. Adding dedicated MY_COLLATION_HANDLERs for collations with no contractions
   (for utf8 and for utf8mb4 character sets). The choice between
   the full-featured handler and the "no contraction" handler is
   made at the collation initialization time.

row_ins_clust_index_entry_low(): Do not call dtuple_t::trim()
before row_ins_clust_index_entry_by_modify(), so that the values
of all columns will be available in row_upd_build_difference_binary().
If applicable, the tuple can be trimmed in btr_cur_optimistic_update()
or btr_cur_pessimistic_update(), which will be called by
row_ins_clust_index_entry_by_modify().

The setting innodb_safe_truncate=ON reduces compatibility with older
versions of MariaDB and backup tools in two ways.

First, we will be writing TRX_UNDO_RENAME_TABLE records, which older
versions do not know about. These records could be misinterpreted if
a DDL transaction was recovered and would be rolled back.
Such rollback is only possible if the server was killed while
an incomplete DDL transaction was persisted. On transaction completion,
the insert_undo log pages would only be repurposed for new undo log
allocations, and their contents would not matter. So, older versions
will not have a problem with innodb_safe_truncate=ON if the server was
shut down cleanly.

Second, to prevent such recovery failure, innodb_safe_truncate=ON will
cause a modification of the redo log format identifier, which will
prevent older versions from starting up after a crash. MariaDB Server
versions older than 10.2.13 will refuse to start up altogether, even
after clean shutdown.

A server restart with innodb_safe_truncate=OFF will restore compatibility
with older server and backup versions.

Added only test case because the bug was fixed by the patch for mdev-17382.

Added only test case because the bug was fixed by the patch for mdev-16992.

The function and_new_conditions_to_optimized_cond() incorrectly handled
the WHERE conditions with one multiple equality and one IN subquery predicate
that could be converted into a jtbm semi-join. This could cause crashes.

The fix code was prepared by Galina Shalygina.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
If the whole page cannot be emptied, do delete the last user record.

btr_pcur_store_position(): Allow a logically empty page contain
a metadata record for generic ALTER TABLE.

try fix OSX build

The syntax error happened because we had not implemented a different print for
percentile functions. The syntax is a bit different when we use percentile functions
as window functions in comparision to normal window functions.
Implemented a seperate print function for percentile functions