Periodically refresh Cassandra cluster topology

[drolando]

Afaict lua-cassandra doesn't automatically detect changes in the ring topology. You have to manually call cluster:refresh() for it to pick up any change.

This is a problem when we're replacing nodes in the C* cluster since the driver will keep trying to connect to the old ones and ignore any new host.

We should just call refresh() even N seconds, ideally after the response has been returned.