sadly we need both a synchronous and an asynchronous promise for this
since destructors cannot be async. we also cannot use forked promises
since multiple threads may be waiting for an auto-gc to complete, but
forked promises are bound to an event loop (and thus a single thread)
Change-Id: I7b4fdbd229c4a1e01bbf80858e93347024f2e333
eldritch horrors
a987d92bd0