Validate empty cell names from counter updates

patch by Aleksey Yeschenko; reviewed by Jeremiah Jordan for
CASSANDRA-7798
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/Constants.java
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/Constants.java
	src/java/org/apache/cassandra/cql3/UpdateParameters.java