Make ServerState announcements work better

[borzunov]

• Before this PR, ServerState.JOINING was announced only once. This announcement quickly expires in case of the full-size BLOOM, since loading blocks takes several minutes. This PR fixes it, so ServerState.JOINING is announced periodically in a thread until blocks are loaded.

• This PR also makes the Server class a non-thread, so it runs in the main thread and can catch KeyboardInterrupt. This is important, since if we are downloading blocks right now, we need to stop it and send the ServerState.OFFLINE message. Note that ModuleContainer is still a thread.

• (minor) For the sake of readability, I moved the ModuleContainer.create() definition, so it is now defined before Server.__init__() (this is because .create() is invoked first).