enum trx_dict_op: dictionary operation modes

trx_get_dict_operation(), trx_set_dict_operation(): Accessors for
trx->dict_operation.

lock_table_enqueue_waiting(), lock_rec_enqueue_waiting(): Do not complain
about lock waits if the dictionary mode is TRX_DICT_OP_INDEX_MAY_WAIT.

row_merge_lock_table(): Remove the work-around for avoiding the warning
in lock_table_enqueue_waiting().

trx_undo_mark_as_dict_operation(): Do not write trx->table_id to the
undo log unless the dict_operation is TRX_DICT_OP_TABLE.

ha_innobase::add_index(): Set the dict_operation mode initially to
TRX_DICT_OP_INDEX_MAY_WAIT, then lock the table exclusively, and set the
mode to TRX_DICT_OP_INDEX, and optionally to TRX_DICT_OP_TABLE when
creating a temporary table.