Avoid overlaps in LCS by not skipping levels and verifying that the candidates
don't overlap any compacting sstables.

Patch by marcuse, reviewed by jbellis for CASSANDRA-6688
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/compaction/LeveledManifest.java