Simplify context

[RazerM]

Logbook has a ContextObject which is used by various objects, most notably the Handler class. As a context manager, these objects will push either greenlet or thread context depending on whether "gevent" support is enabled. There are explicit contextmanagers, push, and pop methods for each variant:

• applicationbound(), push_application(), pop_application()
• threadbound(), push_thread(), pop_thread()
• greenletbound(), push_greenlet(), pop_greenlet()
• contextbound(), push_context(), pop_context()

application context is global and so still has a purpose.

contextvars is available in all the Python versions we support, and greenlet 1.0+ supports it natively, so we can get rid of the thread/greenlet variants.

I wonder if we can already deprecate them and make them use context underneath.