BY THE LOCK

Problem:
FTS SYNC write data from fts index cache to the FTS auxiliary index
table and clear the cache (word by word) at the end.
when FTS SYNC thread is freeing a word, if at the same time, user
read from INFORMATION_SCHEMA.INNODB_FT_INDEX_CACHE. then we may try
read a word from the cache which has been freed.

Fix:
Before start filling INFORMATION_SCHEMA.INNODB_FT_INDEX_CACHE table,
if FTS SYNC is in progress then we will wait for SYNC to finish.

RB: 25079
Reviewed by : Rahul Agarkar <rahul.agarkar@oracle.com>