Don't clear out range tombstones during compaction.

Patch by rlow; reviewed by marcuse for CASSANDRA-7808
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/db/compaction/LazilyCompactedRow.java
	test/unit/org/apache/cassandra/db/RangeTombstoneTest.java