RangeTombstones handled incorrectly in Thrift

Currently a number of thrift tests fail due to a NPE

test_range_deletion 
test_batch_mutate_remove_slice_standard
test_batch_mutate_remove_slice_of_entire_supercolumns

These errors all stem from the fact we aren't properly throwing a validation exception when a deletion with a slice that is not start == finish.

We should add better validation to throw a clear exception sooner.