Add ``key: Callable[..., Hashable]`` support to ``.trionics.maybe_open_context()``

Gives users finer grained control over cache hit behaviour using
a callable which receives the input ``kwargs: dict``. Further this patch
avoids use of the internal ``Actor._service_n`` for spawning the
entering manager tasks and instead allocates and uses an actor global
singleton ``_Cache.service_n``.