2.0 compatibility modifications for CASSANDRA-6931

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6931
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/BatchlogManager.java
Serialize batchlog mutations with the version of the target node

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6931
Add missing CHANGES.txt entry for CASSANDRA-6931
Optimize CounterColumn#reconcile()

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6953
Remove 1.2 sstable support in 2.1

patch by slebresne; reviewed by iamaleksey for CASSANDRA-6869
Merge branch 'cassandra-2.1' of https://git-wip-us.apache.org/repos/asf/cassandra into cassandra-2.1

Conflicts:
	CHANGES.txt
	NEWS.txt