Improve repair tasks(snapshot, differencing) concurrency

patch by yukim; reviewed by sankalp kohli for CASSANDRA-6566
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/repair/RepairJob.java
	src/java/org/apache/cassandra/repair/RepairSession.java
Merge branch 'cassandra-2.1' into trunk