Add property to disable stcs in L0

Patch by marcuse; reviewed by tjake for CASSANDRA-6621
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	NEWS.txt
	src/java/org/apache/cassandra/db/compaction/LeveledManifest.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1