Add an --expire option to the server to age out stale stats after a TTL has been reached

Intended for use with "ephemeral" stats that only live for a finite time.  It is up the the receiver of the stats to figure out what to do when stats stop showing up.

Currently, if a given stat stops showing up, pystatsd will continue publishing the last received value.  The only way to clear "dead" stats is to restart the server.  This change introduces an "expire" option, where old stats will get pruned out after a given TTL, e.g., 300 seconds.