Server.close() hangs for several seconds/minutes

When calling Server.close(), the thread will hang anywhere from a few seconds to 10+ minutes. On older, slower computers, the hang can last over half an hour. Client.close() has a similar problem but will only hang up to a few seconds. Occasionally, the hang ends with a ClosedSelectorException. This problem was introduced in a7e966136798b8e8cd8898bc461e014ed4029d09, which was a fix to #93. Tested on Linux (Lubuntu) using the ChatServer example.