Fix LCS L0 compaction may overlap in L1

patch by yukim; reviewed by jbellis for CASSANDRA-5907
Merge branch 'cassandra-1.2' into cassandra-2.0.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.0.0' into cassandra-2.0
Merge branch 'cassandra-2.0' into trunk