Fixed handling of non-intersecting ranges in anticompaction

patch by Mikhail Stepura; reviewed by Yuki Morishita for CASSANDRA-7892
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	test/unit/org/apache/cassandra/db/compaction/AntiCompactionTest.java