This patch makes Vervis temporarily unusable, because all actors' inbox POST
handlers use the new system, but the actual federation handler code hasn't been
ported. The next patches will port all the S2S activities supported so far, as
well as C2S.
Giving access now starts with an Invite activity, followed by Accept from the
Invite's recipient. Finally, the resource sends a Grant, which is the actual
OCap.
In Haskell by default if a thread has an exception, the main thread isn't
notified at all. This patch changes service thread launching to re-throw their
exceptions in the main thread, so that their failure is noticed.
