Fix recovery from connection error

- Stub the correct Cql method to throw an error (no such method
client#execute_with_consistency).
- Delete both the client and the raw client, so both will be recreated
on retry.
- Move retry logic to #execute_with_consistency.
- Add configurable delay to retry loop.
Merge pull request #176 from clarkbremer/reconnect-to-cassandra

Fix recovery from connection error