Improve termination of Swift servants in response to signals

[pepone]

The pattern used in IceStorm/clock subscriber, isn't simple and also is not thread safe, there is a chance for exit call run when GDC global queue is still releasing the communicator reference in the signal event handler

ice-demos/swift/IceStorm/clock/Sources/Subscriber/main.swift

Lines 41 to 48 in d3d8881

	let sigintSource = DispatchSource.makeSignalSource(signal: SIGINT,
	queue: DispatchQueue.global())
	let sigtermSource = DispatchSource.makeSignalSource(signal: SIGTERM,
	queue: DispatchQueue.global())
	sigintSource.setEventHandler { communicator.shutdown() }
	sigtermSource.setEventHandler { communicator.shutdown() }
	sigintSource.resume()
	sigtermSource.resume()

For the publisher we do all the work in the main thread including the signal handling, but it is still not straightforward and will be good if we come up with a better solution

ice-demos/swift/IceStorm/clock/Sources/Publisher/main.swift

Lines 35 to 45 in d3d8881

	let sigintSource = DispatchSource.makeSignalSource(signal: SIGINT, queue: DispatchQueue.main)
	sigintSource.setEventHandler {
	communicator.destroy()
	}
	sigintSource.resume()
	let sigtermSource = DispatchSource.makeSignalSource(signal: SIGTERM, queue: DispatchQueue.main)
	sigtermSource.setEventHandler {
	communicator.destroy()
	}
	sigtermSource.resume()