ha_innobase::add_index(): Add assertions about !trx->sync_cb and
!trx->dict_redo_list.  Remove a bogus comment.  Replace
row_lock_table_for_merge() with row_merge_lock_table().

row_merge_lock_table(): Renamed from row_lock_table_for_merge()
and moved from row0mysql.c to row0merge.c.  Always lock the table in
the mode LOCK_X.  Call que_graph_free() in order not to leak memory.