Partially https://github.com/sigp/lighthouse/issues/6291
This PR removes the reprocess event channel from being externally exposed. All work events are now sent through the single `BeaconProcessorSend` channel. I've introduced a new `Work::Reprocess` enum variant which we then use to schedule jobs for reprocess. I've also created a new scheduler module which will eventually house the different scheduler impls.
This is all needed as an initial step to generalize the beacon processor
A "full" implementation for the generalized beacon processor can be found here
https://github.com/sigp/lighthouse/pull/6448
I'm going to try to break up the full implementation into smaller PR's so it can actually be reviewed
