Let scrub optionally skip broken counter partitions

patch by Tyler Hobbs; reviewed by Aleksey Yeschenko for CASSANDRA-5930
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/tools/NodeCmd.java
	src/resources/org/apache/cassandra/tools/NodeToolHelp.yaml