commitlog: ignore commitlog segments generated by Cassandra-derived tools
commitlog: ignore commitlog segments generated by Cassandra-derived tools

Cassandra-derived tools (such as sstable2json) may write commitlog segments,
that Scylla cannot recognize.  Since we now write them with a distinct name,
we can recognize the name and ignore these segments, as we know the data they
contain is not interesting.

Fixes #1112.
Message-Id: <5f03d3f9158009abccd6f3816458798c0a67a1bd@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla