Fix validator and in-memory sort ordering

FakeCassandra doesn't honor sort order so any operations that require a
sort then scan will fail. I tried, in vain, to get cassandra running
in-process for the tests. Unfortunately, cassie depends on libthrift
0.5.0 while cassandra requires libthrift 0.9.1. The two are not binary
compatible.

Cassie has been mostly abandoned. It may behove us to implement
zipkin-cassandra directly on top of scrooge generated thrift bindings to
cassandra. That's a larger change for a different time.