Fix counters in supercolumns during live upgrades from 1.2

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-7188
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/CounterUpdateColumn.java
Merge branch 'cassandra-2.1' into trunk