This is preparation for MDEV-12288, which would set DB_TRX_ID=0
when purging history. Also with that change in place, delete-marked
records must always refer to an undo log record via a nonzero
DB_TRX_ID column. (The DB_TRX_ID is only present in clustered index
leaf page records.)

btr_cur_parse_del_mark_set_clust_rec(), rec_get_trx_id():
Statically allocate the offsets
(should never use the heap). Add some debug assertions.

Replace some use of rec_get_trx_id() with row_get_rec_trx_id().

trx_undo_report_row_operation(): Add some sanity checks that are
common for all operations that produce undo log.