Switch to using the OnionMessenger directly in BP

When `OnionMessenger` first developed a timer and events interface,
we accessed the `OnionMessenger` indirectly via the `PeerManager`.
While this is a fairly awkward interface, it avoided a large pile
of generics on the background processor interfaces. However, since
we now have an `AOnionMessenger` trait, this concern is no longer
significant. Further, because we now want to use the built-in
`OnionMessenger` async event processing method, we really need a
direct referene to the `OnionMessenger` in the background
processor, which we add here optionally.

Author: TheBlueMatt