Reduce level of log from debug to trace in CommitLogSegmentManager.java

AbstractCommitLogSegmentManager.java:112
It's changed to trace() in cassandra-3.0 with CASSANDRA-10241:https://github.com/pauloricardomg/cassandra/commit/3ef1b18fa76dce7cd65b73977fc30e51301f3fed#diff-d07279710c482983e537aed26df80400

but not in cassandra-3.11 and trunk. I think it makes sense to make them consistent and downgrade to trace().