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/task_spawner.rs

@@ -30,6 +30,7 @@ impl Priority {
 }
 
 /// Spawns tasks on the `BeaconProcessor` or directly on the tokio executor.
+#[derive(Clone)]
 pub struct TaskSpawner<E: EthSpec> {
     /// Used to send tasks to the `BeaconProcessor`. The tokio executor will be
     /// used if this is `None`.
@@ -155,6 +156,32 @@ impl<E: EthSpec> TaskSpawner<E> {
                 .and_then(|x| x)
         }
     }
+
+    pub fn try_send(&self, work_event: WorkEvent<E>) -> Result<(), warp::Rejection> {
+        if let Some(beacon_processor_send) = &self.beacon_processor_send {
+            let error_message = match beacon_processor_send.try_send(work_event) {
+                Ok(()) => None,
+                Err(TrySendError::Full(_)) => {
+                    Some("The task was dropped. The server is overloaded.")
+                }
+                Err(TrySendError::Closed(_)) => {
+                    Some("The task was dropped. The server is shutting down.")
+                }
+            };
+
+            if let Some(error_message) = error_message {
+                return Err(warp_utils::reject::custom_server_error(
+                    error_message.to_string(),
+                ));
+            };
+
+            Ok(())
+        } else {
+            Err(warp_utils::reject::custom_server_error(
+                "The beacon processor is unavailable".to_string(),
+            ))
+        }
+    }
 }
 
 /// Send a task to the beacon processor and await execution.