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.
Seems like 10 is a pretty big default, no? I would actually say let's have no retries by default, or if it makes sense to have a non-zero default, something in the ballpark of 2 or 3 seems better…

Also `configuration.fetch(:"max-retries", 10)` would be a bit better I think