* net: gracefully disconnect better from remote

This adds in a step to gracefully disconnect (send a "disconnect"
message and then await the flush) before closing the workbench. In some
cases, like Remote - SSH, sending messages is more async than others. In
the exec server connection we handle a `zlib.flate` stream to compress
data to and from the VS Code server, and its API is asynchronous, so
this lets us ensure the stream is drained before giving the go-ahead
to close up shop

This lifecycle phase is a little awkward and I ended it putting it
directly in the lifecycle service: we don't want to do this in
`onWillShutdown` because some contributions want to save data to the
remote workspace, and if shutdown is vetoed it would leave a broken
state. But `onDidShutdown` is synchronous and already depended upon by
several other points, and changing that also felt a bit risky.

cc @alexdima

Refs #211462, will require some small adoption in Remote - SSH to close.

* undo unrelated change

* add priority to joiners

* some cleanup

* cleanup

* tweaks

* `runWithFakedTimers`

* comment

* 💄



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Co-authored-by: Benjamin Pasero <benjamin.pasero@microsoft.com>