Always merge ranges owned by a single node in DES

Patch by Ala' Alkhaldi; review by Tyler Hobbs for CASSANDRA-6930
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk