Start addressGenerator thread on demand

[g1itch]

The addressGenerator thread is started upon PyBitmessage start and uses some resources even if user doesn't create any addresses. I guess it can be refactored to start when it's needed first time to generate an address.

[g1itch]

Or maybe merge addressGenerator with singleWorker. Address generation is a sort of work too.

[g1itch]

Now I see it as part of the objectProcessor thread, or rather a method (or group of methods) of KeyManager() inside the thread.