## Proposed Changes
`@potuz` on the Eth R&D Discord observed that Lighthouse blocks on Pyrmont were always arriving at other nodes after at least 1 second. Part of this could be due to processing and slow propagation, but metrics also revealed that the Lighthouse nodes were usually taking 400-600ms to even just produce a block before broadcasting it.
I tracked the slowness down to the lack of a pre-built tree hash cache (THC) on the states being used for block production. This was due to using the head state for block production, which lacks a THC in order to keep fork choice fast (cloning a THC takes at least 30ms for 100k validators). This PR modifies block production to clone a state from the snapshot cache rather than the head, which speeds things up by 200-400ms by avoiding the tree hash cache rebuild. In practice this seems to have cut block production time down to 300ms or less. Ideally we could _remove_ the snapshot from the cache (and save the 30ms), but it is required for when we re-process the block after signing it with the validator client.
## Alternatives
I experimented with 2 alternatives to this approach, before deciding on it:
* Alternative 1: ensure the `head` has a tree hash cache. This is too slow, as it imposes a +30ms hit on fork choice, which currently takes ~5ms (with occasional spikes).
* Alternative 2: use `Arc<BeaconSnapshot>` in the snapshot cache and share snapshots between the cache and the `head`. This made fork choice blazing fast (1ms), and block production the same as in this PR, but had a negative impact on block processing which I don't think is worth it. It ended up being necessary to clone the full state from the snapshot cache during block production, imposing the +30ms penalty there _as well_ as in block production.
In contract, the approach in this PR should only impact block production, and it improves it! Yay for pareto improvements 🎉
## Additional Info
This commit (ac59dfa) is currently running on all the Lighthouse Pyrmont nodes, and I've added a dashboard to the Pyrmont grafana instance with the metrics.
In future work we should optimise the attestation packing, which consumes around 30-60ms and is now a substantial contributor to the total.
## Issue Addressed
NA
## Proposed Changes
- Refactor the `bls` crate to support multiple BLS "backends" (e.g., milagro, blst, etc).
- Removes some duplicate, unused code in `common/rest_types/src/validator.rs`.
- Removes the old "upgrade legacy keypairs" functionality (these were unencrypted keys that haven't been supported for a few testnets, no one should be using them anymore).
## Additional Info
Most of the files changed are just inconsequential changes to function names.
## TODO
- [x] Optimization levels
- [x] Infinity point: https://github.com/supranational/blst/issues/11
- [x] Ensure milagro *and* blst are tested via CI
- [x] What to do with unsafe code?
- [x] Test infinity point in signature sets
