Bucket.refresh() asks the new bucket info from a random server now.

If the chosen random server fails, it tries the next one, and loops over the
nodes list until it tries all the nodes. This way, even if the original node
that the couchbase client connected to dies, everything would keep working.

This addresses the issue that I raised here: https://github.com/couchbaselabs/go-couchbase/issues/26