Add option to do more aggressive tombstone compaction.

Patch by pauloricardomg; reviewed by marcuse for CASSANDRA-6563
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	test/unit/org/apache/cassandra/db/compaction/CompactionsTest.java
Merge branch 'cassandra-2.1' into trunk