Add DC-aware sequential repair

patch by Jimmy Mårdell; reviewed by yukim for CASSANDRA-8193
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
	src/java/org/apache/cassandra/service/ActiveRepairService.java
	src/java/org/apache/cassandra/service/StorageService.java
	src/java/org/apache/cassandra/service/StorageServiceMBean.java
	src/java/org/apache/cassandra/tools/NodeCmd.java
	src/java/org/apache/cassandra/tools/NodeProbe.java
	src/resources/org/apache/cassandra/tools/NodeToolHelp.yaml