Fix truncate to always call flush on table

Patch by Jeremiah Jordan; reviewed by tjake for (CASSANDRA-7511)
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/db/DataTracker.java
	test/unit/org/apache/cassandra/db/CommitLogTest.java
Fix min/max cell name collection with 2.0 range tombstones

Patch by Tyler Hobbs; review by Marcus Eriksson for CASSANDRA-7593