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/client/src/compute_light_client_updates.rs

@@ -1,8 +1,8 @@
 use beacon_chain::{BeaconChain, BeaconChainTypes, LightClientProducerEvent};
 use beacon_processor::work_reprocessing_queue::ReprocessQueueMessage;
+use beacon_processor::{BeaconProcessorSend, Work, WorkEvent};
 use futures::channel::mpsc::Receiver;
 use futures::StreamExt;
-use tokio::sync::mpsc::Sender;
 use tracing::error;
 
 // Each `LightClientProducerEvent` is ~200 bytes. With the light_client server producing only recent
@@ -14,7 +14,7 @@ pub(crate) const LIGHT_CLIENT_SERVER_CHANNEL_CAPACITY: usize = 32;
 pub async fn compute_light_client_updates<T: BeaconChainTypes>(
     chain: &BeaconChain<T>,
     mut light_client_server_rv: Receiver<LightClientProducerEvent<T::EthSpec>>,
-    reprocess_tx: Sender<ReprocessQueueMessage>,
+    beacon_processor_send: BeaconProcessorSend<T::EthSpec>,
 ) {
     // Should only receive events for recent blocks, import_block filters by blocks close to clock.
     //
@@ -31,7 +31,13 @@ pub async fn compute_light_client_updates<T: BeaconChainTypes>(
             });
 
         let msg = ReprocessQueueMessage::NewLightClientOptimisticUpdate { parent_root };
-        if reprocess_tx.try_send(msg).is_err() {
+        if beacon_processor_send
+            .try_send(WorkEvent {
+                drop_during_sync: true,
+                work: Work::Reprocess(msg),
+            })
+            .is_err()
+        {
             error!(%parent_root,"Failed to inform light client update")
         };
     }