Improve handling of range tomsbstones for wide partitions

patch by m0nstermind; reviewed by slebresne for CASSANDRA-6446
Merge branch 'cassandra-2.1' of https://git-wip-us.apache.org/repos/asf/cassandra into cassandra-2.1
Changelog fix
Merge branch 'cassandra-2.1' into trunk
Fix bad merge
Merge branch 'cassandra-2.1' into trunk