Add auto retry mechanism on connection failure

In development, I am often restarting my cassandra clusters and find that the cequel gem would not handle reconnecting gracefully without a manual reconnection.

This pull request provides a retry mechanism to auto-reconnect to cassandra a number of times specified by a `max-retries` config parameter that defaults to 10.  It does this by clearing out the disconnected `raw_client` instance in `Cequel::Metal::Keyspace` when it detects a connection error.

Thanks ahead of time for looking at this and feedback is appreciated.
I don't think it matters&mdash;I can do either it either way.  If you feel strongly about it, I have no problem changing it to an underscore.