ChannelInvoker.newPromise should pass file and line to eventLoop.newPromise

[Lukasa]

We normally want debugging information to keep track of where our promises are allocated in debug modes. However, if someone does Channel.newPromise instead of eventLoop.newPromise, that information is lost. The core issue is here:

swift-nio/Sources/NIO/ChannelInvoker.swift

Lines 182 to 184 in fef728c

	private func newPromise() -> EventLoopPromise<Void> {
	return eventLoop.newPromise()
	}

This indirection should preserve the #file and #line specific debugging logic we use elsewhere.

[tib]

Hey guys, I think I've fixed this with my PR.