thrift and embedded cassandra: enable compression

Now all column families created by the CassandraThrift and
CassandraEmbedded backends enable snappy compression on those
newly-created column families.

Astyanax does not.  Our astyanax version is too old to have the
compression configuration methods required.  Upgrading to the latest
release caused test failures (without any source changes -- just
changing the version of astyanax used in testing).  We'll need to
address that eventually.