If CREATE TABLE...FULLTEXT INDEX was initiated right before shutdown,
then the function fts_load_stopword() could commit modifications
after shutdown was initiated, causing an assertion failure in
the function trx_purge_add_update_undo_to_history().

Mark as internal all the read/write transactions that
modify fulltext indexes, so that they will be ignored by
the assertion that guards against transaction commits
after shutdown has been initiated.

fts_optimize_free(): Invoke trx_commit_for_mysql() just in case,
because in fts_optimize_create() we started the transaction as
internal, and fts_free_for_backgruond() would assert that the
flag is clear. Transaction commit would clear the flag.