Mark hosts up at the right time when adding them

Hosts outside of the intial set of contact points were not
being marked up correctly when they were discovered and added
to the cluster and sessions, resulting in them being skipped
for statement preparation and other operations.
Don't ignored libev conns closed by server

This bug was introduced by 31f684bb91f3e0a41238c869a7c8fc11ebf21515
Minor cleanup in metrics test