Fix https://github.com/zerotier/libzt/issues/242

[bostick]

When processing events, AttachCurrentThread is called, but matching DetachCurrentThread was not being called

This prevented main thread from exiting

[joseph-henry]

Thank you. I really like that you included a test with the commit. I do think we should maybe combine it with the existing java selftest thouth and add some assertions at some point.

[bostick]

Yes. For adding to selftest, would recommend to launch a child process of java ExitTest and check that it terminates. i.e., not relying on the test harness process itself or running as separate thread inside the process.