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/service.rs

@@ -6,7 +6,7 @@ use crate::router::{Router, RouterMessage};
 use crate::subnet_service::{SubnetService, SubnetServiceMessage, Subscription};
 use crate::NetworkConfig;
 use beacon_chain::{BeaconChain, BeaconChainTypes};
-use beacon_processor::{work_reprocessing_queue::ReprocessQueueMessage, BeaconProcessorSend};
+use beacon_processor::BeaconProcessorSend;
 use futures::channel::mpsc::Sender;
 use futures::future::OptionFuture;
 use futures::prelude::*;
@@ -211,7 +211,6 @@ impl<T: BeaconChainTypes> NetworkService<T> {
         executor: task_executor::TaskExecutor,
         libp2p_registry: Option<&'_ mut Registry>,
         beacon_processor_send: BeaconProcessorSend<T::EthSpec>,
-        beacon_processor_reprocess_tx: mpsc::Sender<ReprocessQueueMessage>,
     ) -> Result<
         (
             NetworkService<T>,
@@ -315,7 +314,6 @@ impl<T: BeaconChainTypes> NetworkService<T> {
             executor.clone(),
             invalid_block_storage,
             beacon_processor_send,
-            beacon_processor_reprocess_tx,
             fork_context.clone(),
         )?;
 
@@ -367,7 +365,6 @@ impl<T: BeaconChainTypes> NetworkService<T> {
         executor: task_executor::TaskExecutor,
         libp2p_registry: Option<&'_ mut Registry>,
         beacon_processor_send: BeaconProcessorSend<T::EthSpec>,
-        beacon_processor_reprocess_tx: mpsc::Sender<ReprocessQueueMessage>,
     ) -> Result<(Arc<NetworkGlobals<T::EthSpec>>, NetworkSenders<T::EthSpec>), String> {
         let (network_service, network_globals, network_senders) = Self::build(
             beacon_chain,
@@ -375,7 +372,6 @@ impl<T: BeaconChainTypes> NetworkService<T> {
             executor.clone(),
             libp2p_registry,
             beacon_processor_send,
-            beacon_processor_reprocess_tx,
         )
         .await?;