This is a backport of the following fix from MySQL 5.7.23.
Some code refactoring has been omitted, and the test case has
been adapted to MariaDB.

commit 7a689acaa65e9d602575f7aa53fe36a64a07460f
Author: Krzysztof Kapuścik <krzysztof.kapuscik@oracle.com>
Date:   Tue Mar 13 12:34:03 2018 +0100

Bug#27082268 Invalid FTS sync synchronization

The fix closes two issues:
Bug #27082268 - INNODB: FAILING ASSERTION: SYM_NODE->TABLE != NULL DURING FTS SYNC
Bug #27095935 - DEADLOCK BETWEEN FTS_DROP_INDEX AND FTS_OPTIMIZE_SYNC_TABLE

Both issues were related to a FTS cache sync being done during
operations that perfomed DDL actions on internal FTS tables
(ALTER TABLE, TRUNCATE). In some cases the FTS tables and/or
internal cache structures could get removed while still being
used to perform FTS synchronization leading to crashes. In other
the sync operations could not get finishes as it was waiting for
dict lock which was taken by thread waiting for the background
sync to be finished.

The changes done includes:
- Stopping background operations during ALTER TABLE and TRUNCATE.
- Removal of unused code in FTS.
- Cleanup of FTS sync related code to make it more readable and
easier to maintain.

RB#18262