`QuiescingHelper` init & forget will crash your program

[weissi]

swift-nio-extras/Sources/NIOExtras/QuiescingHelper.swift

Line 229 in a75e92b

self.channelCollectorPromise = group.next().makePromise()

If you create a new QuiescingHelper() but then end up not using it, it'll crash your program for a leaked promise. There's probably a better solution for that. If it should remain the API contract we can probably give the user a better message than the leaked promise?

[Lukasa]

Hmm, the API contract absolutely requires that you call makeServerChannelHandler exactly once. We can probably enforce that constraint using an atomic bool or a lock.