Fix LIMIT with static columns

patch by slebresne; reviewed by iamaleksey for CASSANDRA-6956
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/filter/ColumnCounter.java
Fix 6956 patch
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/filter/ColumnCounter.java