Remove reprocess channel (#7437)

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

beacon_node/network/src/service/tests.rs

@@ -58,8 +58,6 @@ fn test_dht_persistence() {
         let BeaconProcessorChannels {
             beacon_processor_tx,
             beacon_processor_rx: _beacon_processor_rx,
-            work_reprocessing_tx,
-            work_reprocessing_rx: _work_reprocessing_rx,
         } = <_>::default();
 
         let _network_service = NetworkService::start(
@@ -68,7 +66,6 @@ fn test_dht_persistence() {
             executor,
             None,
             beacon_processor_tx,
-            work_reprocessing_tx,
         )
         .await
         .unwrap();
@@ -137,7 +134,6 @@ fn test_removing_topic_weight_on_old_topics() {
             executor.clone(),
             None,
             beacon_processor_channels.beacon_processor_tx,
-            beacon_processor_channels.work_reprocessing_tx,
         )
         .await
         .unwrap()