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/tests/interactive_tests.rs

@@ -4,7 +4,7 @@ use beacon_chain::{
     test_utils::{AttestationStrategy, BlockStrategy, LightClientStrategy, SyncCommitteeStrategy},
     ChainConfig,
 };
-use beacon_processor::work_reprocessing_queue::ReprocessQueueMessage;
+use beacon_processor::{work_reprocessing_queue::ReprocessQueueMessage, Work, WorkEvent};
 use eth2::types::ProduceBlockV3Response;
 use eth2::types::{DepositContractData, StateId};
 use execution_layer::{ForkchoiceState, PayloadAttributes};
@@ -923,14 +923,16 @@ async fn queue_attestations_from_http() {
         .unwrap();
     tester
         .ctx
-        .beacon_processor_reprocess_send
+        .beacon_processor_send
         .as_ref()
         .unwrap()
-        .send(ReprocessQueueMessage::BlockImported {
-            block_root,
-            parent_root,
+        .try_send(WorkEvent {
+            drop_during_sync: false,
+            work: Work::Reprocess(ReprocessQueueMessage::BlockImported {
+                block_root,
+                parent_root,
+            }),
         })
-        .await
         .unwrap();
 
     attestation_future.await.unwrap();