Fix handling of empty counter replication mutations

patch by Aleksey Yeschenko; reviewed by Richard Low for CASSANDRA-7144
CASSANDRA-7144 follow-up
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/service/StorageProxy.java
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/CounterMutation.java
	src/java/org/apache/cassandra/service/StorageProxy.java