Don't emit the 'close' event with process.nextTick.
Closing a handle is an operation that usually *but not always* completes
on the next tick of the event loop, hence using process.nextTick is not
reliable.
Use a proper handle close callback and emit the 'close' event from
inside the callback.
Update tests that depend on the intricacies of the old model.
Fixes#3459.
The server 'close' event was emitted before the last client 'close' event. Not
exactly fatal but potentially confusing.
Before this commit the order looked something like [client, server, client],
now it looks like [client, client, server].
See #3340 for more details.