fts_cmp_set_sync_doc_id(), fts_load_stopword(): Start the transaction
in read-only mode if innodb_read_only is set.

fts_update_sync_doc_id(), fts_commit_table(), fts_sync(),
fts_optimize_table(): Return DB_READ_ONLY if innodb_read_only is set.

fts_doc_fetch_by_doc_id(), fts_table_fetch_doc_ids():
Remove the code to start an internal transaction or to roll back,
because this is a read-only operation.