Problem:
========
MLOG_FILE_WRITE_CRYPT_DATA redo log fails to apply type for
the crypt_data present in the space. While processing the double-write
buffer pages, page fails to decrypt. It leads to warning message.

Fix:
====
Set the type while parsing MLOG_FILE_WRITE_CRYPT_DATA redo log.
If type and length is of invalid type then mark it as corrupted.

json_t unittest added.

JSON api implementations and tests pushed.
sql_acl.cc fixed with the new function names.

Corrects 94d722b6

- There is no need to wait for crypt thread to stop accessing space while
dropping the garbage encrypted tablespace during recover.

dict_table_t::init_instant(): Correctly initialize the length of
variable-length instantly dropped columns.

row_ins_index_entry_set_vals(): For variable-length instantly dropped
columns, write 0 bytes of data. For dropped fixed-length NOT NULL
columns, write the fixed length of NUL bytes as data.

Allow table definition cache be bigger than open table cache (due to problem with VIEWs and prepared statements).

Implement according to standard SQL specification 2008.
The check_constraints table is used for fetching metadata about
the constraints defined for tables in all databases.

There were some result files which failed after running mtr.
These files are updated with newly create record with mtr --record.

This might apply to 10.1 and 10.2 as well, but I cannot run that test
there without installing third-party software. The test would expect
"innobackupex" to be in path. If I add a wrapper script that invokes
mariabackup --innobackupex, then on 10.2 the cluster would fail to start
up due to a version number mismatch.

MDEV-18173 Assertion `o->ind == vers_end' or `o->ind == vers_start' failed in dict_table_t::instant_column

Poisoning memory after munmap() and friends is totally incorrect as this memory
could be anything.

os_mem_free_large(): remove memory poisoning

If galera.galera_gtid_slave_sst_rsync is repeated more than once it will fail due incorrect GTID position. After stopping SLAVE node reset also GTID_SLAVE_POS variable.

The merge commit d833bb65 did not
correctly merge the commit 03eb1593.

Closes #948

ha_innobase_inplace_ctx::create_key_defs(): Replaces
innobase_create_key_defs().

dict_index_t::contains_col_or_prefix(): Replaces
dict_index_contains_col_or_prefix().

Closes #988

This mutex can be freed when server shuts down (when thread_count goes down to 0)
, but it is still used inside THD::~THD() when Statement_map is destroyed.

The fix is to call Statement_map::reset() at the point where thread_count
is still positive, and avoid locking LOCK_prepared_stmt_count in THD
destructor.

Add the test case. The actual bug was fixed in MDEV-17541.

Closes #811

When performing a hash search via HASH_SEARCH we first look at a key of a node
and then at its pointer to the next node in chain. If we have those in one cache
line instead of a two we reduce memory reads.

I found dict_table_t, fil_space_t and buf_page_t suitable for such improvement.

Bug was introduced in this commit:

  commit: a9ca8198
      Call alloc() instead of realloc()
      Use alloc() if we don't need original string (avoid copy)
      Removed not needed test of str_length in sql_string.cc

copy_if_not_alloced() was forgotten when changing realloc()'s to alloc()'s.
Changing it now.