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
assert renew memtable is only used when durable writes = false
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
(cqlsh) Fix DESCRIBE for NTS keyspaces

patch by Tyler Hobbs; reviewed by Aleksey Yeschenko for CASSANDRA-7729
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
fix cl unit tests