sstables: mp_row_consumer_m: Add trace-level logging
sstables: mp_row_consumer_m: Add trace-level logging

Very useful for debugging. The old mp_row_consumer_k_l had this.

Message-Id: <7ee9fa5980ce7158972ee9a3612dc8c2103723a8@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,avikivity/scylla