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.
Merge pull request #63 from 0xDEC0DE/master

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