Add back multi server for node platform

[natebosch]

Fixes #1278

Copy the _loopback code from package:multi_server_socket but model
it as a List<ServerSocket> instead of a single server. Remove the
unnecessary arguments for handling anything other than port 0.

Use a finally block to ensure the socket servers are always closed.

[natebosch]

It looks like the error is hidden from us when it happens by

test/pkgs/test_api/lib/src/backend/invoker.dart

Lines 355 to 357 in 58383da

	// However, users don't think of load tests as "tests", so the error isn't
	// helpful for them.
	if (liveTest.suite.isLoadSuite) return;

I can't figure out why an error swallowed there ends up making the runner hang on Travis though, but it does seem to do that.