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/router.rs

@@ -10,9 +10,7 @@ use crate::service::NetworkMessage;
 use crate::status::status_message;
 use crate::sync::SyncMessage;
 use beacon_chain::{BeaconChain, BeaconChainTypes};
-use beacon_processor::{
-    work_reprocessing_queue::ReprocessQueueMessage, BeaconProcessorSend, DuplicateCache,
-};
+use beacon_processor::{BeaconProcessorSend, DuplicateCache};
 use futures::prelude::*;
 use lighthouse_network::rpc::*;
 use lighthouse_network::{
@@ -87,7 +85,6 @@ impl<T: BeaconChainTypes> Router<T> {
         executor: task_executor::TaskExecutor,
         invalid_block_storage: InvalidBlockStorage,
         beacon_processor_send: BeaconProcessorSend<T::EthSpec>,
-        beacon_processor_reprocess_tx: mpsc::Sender<ReprocessQueueMessage>,
         fork_context: Arc<ForkContext>,
     ) -> Result<mpsc::UnboundedSender<RouterMessage<T::EthSpec>>, String> {
         trace!("Service starting");
@@ -103,7 +100,6 @@ impl<T: BeaconChainTypes> Router<T> {
             chain: beacon_chain.clone(),
             network_tx: network_send.clone(),
             sync_tx: sync_send.clone(),
-            reprocess_tx: beacon_processor_reprocess_tx,
             network_globals: network_globals.clone(),
             invalid_block_storage,
             executor: executor.clone(),