Fix replaying pre-2.0 commit logs

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6714
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/commitlog/CommitLogDescriptor.java
	src/java/org/apache/cassandra/db/commitlog/CommitLogReplayer.java