Move the BeaconProcessor into a new crate (#4435)

*Replaces #4434. It is identical, but this PR has a smaller diff due to a curated commit history.*

## Issue Addressed

NA

## Proposed Changes

This PR moves the scheduling logic for the `BeaconProcessor` into a new crate in `beacon_node/beacon_processor`. Previously it existed in the `beacon_node/network` crate.

This addresses a circular-dependency problem where it's not possible to use the `BeaconProcessor` from the `beacon_chain` crate. The `network` crate depends on the `beacon_chain` crate (`network -> beacon_chain`), but importing the `BeaconProcessor` into the `beacon_chain` crate would create a circular dependancy of `beacon_chain -> network`.

The `BeaconProcessor` was designed to provide queuing and prioritized scheduling for messages from the network. It has proven to be quite valuable and I believe we'd make Lighthouse more stable and effective by using it elsewhere. In particular, I think we should use the `BeaconProcessor` for:

1. HTTP API requests.
1. Scheduled tasks in the `BeaconChain` (e.g., state advance).

Using the `BeaconProcessor` for these tasks would help prevent the BN from becoming overwhelmed and would also help it to prioritize operations (e.g., choosing to process blocks from gossip before responding to low-priority HTTP API requests).

## Additional Info

This PR is intended to have zero impact on runtime behaviour. It aims to simply separate the *scheduling* code (i.e., the `BeaconProcessor`) from the *business logic* in the `network` crate (i.e., the `Worker` impls). Future PRs (see #4462) can build upon these works to actually use the `BeaconProcessor` for more operations.

I've gone to some effort to use `git mv` to make the diff look more like "file was moved and modified" rather than "file was deleted and a new one added". This should reduce review burden and help maintain commit attribution.

Cargo.toml

@@ -4,6 +4,7 @@ members = [
 
     "beacon_node",
     "beacon_node/beacon_chain",
+    "beacon_node/beacon_processor",
     "beacon_node/builder_client",
     "beacon_node/client",
     "beacon_node/eth1",

beacon_node/Cargo.toml

@@ -41,4 +41,4 @@ monitoring_api = { path = "../common/monitoring_api" }
 sensitive_url = { path = "../common/sensitive_url" }
 http_api = { path = "http_api" }
 unused_port = { path = "../common/unused_port" }
-strum = "0.24.1"
+strum = "0.24.1"

beacon_node/beacon_processor/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "beacon_processor"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+slog = { version = "2.5.2", features = ["max_level_trace"] }
+itertools = "0.10.0"
+logging = { path = "../../common/logging" }
+tokio = { version = "1.14.0", features = ["full"] }
+tokio-util = { version = "0.6.3", features = ["time"] }
+futures = "0.3.7"
+fnv = "1.0.7"
+strum = "0.24.0"
+task_executor = { path = "../../common/task_executor" }
+slot_clock = { path = "../../common/slot_clock" }
+lighthouse_network =  { path = "../lighthouse_network" }
+hex = "0.4.2"
+derivative = "2.2.0"
+types = { path = "../../consensus/types" }
+ethereum_ssz = "0.5.0"
+lazy_static = "1.4.0"
+lighthouse_metrics = { path = "../../common/lighthouse_metrics" }
+parking_lot = "0.12.0"