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/http_api/src/publish_attestations.rs

@@ -40,15 +40,13 @@ use beacon_chain::{
     BeaconChainTypes,
 };
 use beacon_processor::work_reprocessing_queue::{QueuedUnaggregate, ReprocessQueueMessage};
+use beacon_processor::{Work, WorkEvent};
 use eth2::types::Failure;
 use lighthouse_network::PubsubMessage;
 use network::NetworkMessage;
 use std::sync::Arc;
 use std::time::Duration;
-use tokio::sync::{
-    mpsc::{Sender, UnboundedSender},
-    oneshot,
-};
+use tokio::sync::{mpsc::UnboundedSender, oneshot};
 use tracing::{debug, error, warn};
 use types::SingleAttestation;
 
@@ -130,7 +128,7 @@ pub async fn publish_attestations<T: BeaconChainTypes>(
     chain: Arc<BeaconChain<T>>,
     attestations: Vec<SingleAttestation>,
     network_tx: UnboundedSender<NetworkMessage<T::EthSpec>>,
-    reprocess_send: Option<Sender<ReprocessQueueMessage>>,
+    allow_reprocess: bool,
 ) -> Result<(), warp::Rejection> {
     // Collect metadata about attestations which we'll use to report failures. We need to
     // move the `attestations` vec into the blocking task, so this small overhead is unavoidable.
@@ -142,6 +140,7 @@ pub async fn publish_attestations<T: BeaconChainTypes>(
     // Gossip validate and publish attestations that can be immediately processed.
     let seen_timestamp = timestamp_now();
     let mut prelim_results = task_spawner
+        .clone()
         .blocking_task(Priority::P0, move || {
             Ok(attestations
                 .into_iter()
@@ -156,7 +155,7 @@ pub async fn publish_attestations<T: BeaconChainTypes>(
                         Err(Error::Validation(AttestationError::UnknownHeadBlock {
                             beacon_block_root,
                         })) => {
-                            let Some(reprocess_tx) = &reprocess_send else {
+                            if !allow_reprocess {
                                 return PublishAttestationResult::Failure(Error::ReprocessDisabled);
                             };
                             // Re-process.
@@ -180,7 +179,13 @@ pub async fn publish_attestations<T: BeaconChainTypes>(
                                     beacon_block_root,
                                     process_fn: Box::new(reprocess_fn),
                                 });
-                            if reprocess_tx.try_send(reprocess_msg).is_err() {
+                            if task_spawner
+                                .try_send(WorkEvent {
+                                    drop_during_sync: false,
+                                    work: Work::Reprocess(reprocess_msg),
+                                })
+                                .is_err()
+                            {
                                 PublishAttestationResult::Failure(Error::ReprocessFull)
                             } else {
                                 PublishAttestationResult::Reprocessing(rx)