[SR-3199] Cancel handler is never called on DispatchSource after invoking `.cancel()`

Issue Description:

It appears that the cancel handler is never invoked after calling cancel on a DispatchSource in Swift 3 on macOS.

This issue can be replicated by following steps with Swift 3.0.1.

1. Clone https://github.com/SwiftOnEdge/Edge.git
2. Check out the commit 1bc2552c3f7cb0d33080d65b1290008ac5342cdf
3. Comment out line 125 of /Sources/TCP/Server.swift and insert print statement there.
4. Insert a print statement in the cancelHandler on line 47 of the same file
5. Run the test RouterTests.testMiddleware

You should see that although the cancel in invoked, the test completes without ever invoking the cancel handler configured on line 47.

Shouldn't the listeningSource maintain a strong reference to the handler and submit it to the main queue (in this case) to be run when cancel is invoked or am I misunderstanding something about the contract of cancel?

Note: This is on macOS. I have not tested this for Linux.

Reproduce the macOS behavior using the Edge commit 1bc2552c3f7cb0d33080d65b1290008ac5342cdf, focusing on Sources/TCP/Server.swift lines 47 and 125 and RouterTests.testMiddleware. Check the DispatchSource cancellation contract and compare the observed output with the expected cancel-handler behavior; done means the issue is resolved or the platform-specific contract is clearly established by a regression test.