Remove reprocess channel

[eserilev]

Issue Addressed

Partially #6291

Proposed Changes

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
#6448

I'm going to try to break up the full implementation into smaller PR's so it can actually be reviewed

[michaelsproul]

Nice! Love this!

[eserilev]

Since reprocess and regular work events will all come through a single channel, we may need to raise the default work queue size like we're doing here: #7415