Heartbeat timeout means the period length that indicates heartbeat is
out of service, which is different from heartbeat interval.
So we should use '-peer-heartbeat-interval' instead of
'-peer-heartbeat-timeout' in etcd.
'-peer-heartbeat-timeout' is deprecated but still could be used.
Refactor config into its own package. Trying to tease the config from
the server so that all of the control surfaces are exposed in the Server
for easier testing.
