Fixed a race condition when shutting down Agg

[wblakecaldwell]

When the Agg is stopped, the done channel is written to, which signals
its aggregate() goroutine loop to exit. This is fine, except this done
channel is also returned to the caller via Done(). The intended flow
is for aggregate() to catch the done channel write, and then wrap
up, close the output channel that the caller is reading from, and write
again to the done channel for the caller. However, if the caller
catches the first message before Agg does, then Agg never shuts down.

The fix here is to have two channels - one to signal that we're shutting
down (quitting), and one to signal that we've shut down (done). The
first one is private, just for communication with its own goroutines.
The second is exposed to the caller. We're also now closing channels,
rather than writing to them, for good measure, since we're only using
them as quit channels.

[wg]

LGTM

[wblakecaldwell]

thanks