sstables: merge row_consumer into mp_row_consumer_k_l
sstables: merge row_consumer into mp_row_consumer_k_l

The row_consumer interface has only one implementation:
mp_row_consumer_k_l; and we're not planning other ones,
so to reduce the number of inheritances, and the number
of lines in the sstable reader, these classes may be
combined.

Signed-off-by: Wojciech Mitros <a1165404be25af9d11ad8ab7c126b911264f9b80@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla