Add client time to server start time.

Summary: Ideally, `Unix.time () - start_time` would measure the total time it takes to start the
server from the moment the user issues a `hh_client start` command. Previously,
we ignored the time spent in running `hh_client` since that was generally negligible.
However, now that `hh_client` can invoke an external options script, the running time
can actually be noticeable depending on what the external script does; hence, we
should really set `start_time` when `hh_client` starts up, rather than when `hh_server`
gets started.

Reviewed By: @jwatzman

Differential Revision: D1737767

Signature: t1:1737767:1418429942:109e016d9eb54b561f040703355343fb73f42b8f