Problem:
========
  InnoDB FTS requesting the fts sync of the table once the fts
cache size reaches 1/10 of innodb_ft_cache_size. But fts_sync()
releases cache lock when writing the word. By doing this, InnoDB
insert thread increases the innodb fts cache memory and
SYNC operation will take more time to complete.

Solution:
=========
  Remove the fts sync operation(FTS_MSG_SYNC_TABLE) from
the fts optimize background thread. Instead of that,
allow user thread to sync the InnoDB fts cache when
the cache size exceeds 512 kb. User thread holds
cache lock while doing cache syncing, it make sure that
other threads doesn't add the docs into the cache.

Removed FTS_MSG_SYNC_TABLE and its related function
because we do remove the FTS_MSG_SYNC_TABLE message
itself.

Removed fts_sync_index_check() and all related
function because other threads doesn't add while
cache operation going on.