Track min/max timestamps for range tombstones.

Patch by marcuse; reviewed by iamaleksey for CASSANDRA-7647
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
fix merge
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
Avoid NPE when listing saved caches directory

Patch by marcuse; reviewed by yukim for CASSANDRA-7632
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk