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.

Author: @sprsquish
Fixes #342
URL: https://github.com/twitter/zipkin/pull/342