purge_node_t, undo_node_t: Change the type of rec_type and cmpl_info
to byte, because this data is being extracted from a single byte.

UndoRecApplier: Change type and cmpl_info to be of type byte, and
move them next to the 16-bit offset field to minimize alignment bloat.

row_purge_parse_undo_rec(): Remove some redundant code. Purge will
be started by innodb_ddl_recovery_done(), at which point all
necessary subsystems will have been initialized.

trx_purge_rec_t::undo_rec: Point to const.

Reviewed by: Vladislav Lesin