Fix leaking of transports due to connection lost

When a connection is broken the connectionLost method of the transport is
called. The default behaviour is to do nothing. This patch overrides it
to store the new state on the transport. The transport is then garbage
collected the next time _getFreeTransport is called.

Unfortunately I haven't been able to write any tests to prove that this fixes the problem, but I have been testing it out on my own system by stopping and then restarting one of my Riak nodes.