sstables: fold components_writer into sstable_writer_k_l
sstables: fold components_writer into sstable_writer_k_l

It serves no purpose being a different class but being called
by sstable_writer_k_l.

Refs #3012.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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