*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.
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