Validate SliceRange start and finish lengths

patch by Ben Bromhead; reviewed by Aleksey Yeschenko for CASSANDRA-6521
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/thrift/ThriftValidation.java
Merge branch 'cassandra-2.0' into trunk
Fix merge