Do not flush on truncate if durable_writes is false

Patch by Jeremiah Jordan; reviewed by tjake for CASSANDRA-7750
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge 2.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	test/unit/org/apache/cassandra/db/CommitLogTest.java