Optimise attestation selection proof signing (#4033)

## Issue Addressed

Closes #3963 (hopefully)

## Proposed Changes

Compute attestation selection proofs gradually each slot rather than in a single `join_all` at the start of each epoch. On a machine with 5k validators this replaces 5k tasks signing 5k proofs with 1 task that signs 5k/32 ~= 160 proofs each slot.

Based on testing with Goerli validators this seems to reduce the average time to produce a signature by preventing Tokio and the OS from falling over each other trying to run hundreds of threads. My testing so far has been with local keystores, which run on a dynamic pool of up to 512 OS threads because they use [`spawn_blocking`](https://docs.rs/tokio/1.11.0/tokio/task/fn.spawn_blocking.html) (and we haven't changed the default).

An earlier version of this PR hyper-optimised the time-per-signature metric to the detriment of the entire system's performance (see the reverted commits). The current PR is conservative in that it avoids touching the attestation service at all. I think there's more optimising to do here, but we can come back for that in a future PR rather than expanding the scope of this one.

The new algorithm for attestation selection proofs is:

- We sign a small batch of selection proofs each slot, for slots up to 8 slots in the future. On average we'll sign one slot's worth of proofs per slot, with an 8 slot lookahead.
- The batch is signed halfway through the slot when there is unlikely to be contention for signature production (blocks are <4s, attestations are ~4-6 seconds, aggregates are 8s+).

## Performance Data

_See first comment for updated graphs_.

Graph of median signing times before this PR:

![signing_times_median](https://user-images.githubusercontent.com/4452260/221495627-3ab3c105-319f-406e-b99d-b5913e0ded9c.png)

Graph of update attesters metric (includes selection proof signing) before this PR:

![update_attesters_store](https://user-images.githubusercontent.com/4452260/221497057-01ba40e4-8148-45f6-9e21-36a9567a631a.png)

Median signing time after this PR (prototype from 12:00, updated version from 13:30):

![signing_times_median_updated](https://user-images.githubusercontent.com/4452260/221771578-47a040cc-b832-482f-9a1a-d1bd9854e00e.png)

99th percentile on signing times (bounded attestation signing from 16:55, now removed):

![signing_times_99pc](https://user-images.githubusercontent.com/4452260/221772055-e64081a8-2220-45ba-ba6d-9d7e344a5bde.png)

Attester map update timing after this PR:

![update_attesters_store_updated](https://user-images.githubusercontent.com/4452260/221771757-c8558a48-7f4e-4bb5-9929-dee177a66c1e.png)

Selection proof signings per second change:

![signing_attempts](https://user-images.githubusercontent.com/4452260/221771855-64f5da22-1655-478d-926b-810be8a3650c.png)

## Link to late blocks

I believe this is related to the slow block signings because logs from Stakely in #3963 show these two logs almost 5 seconds apart:

> Feb 23 18:56:23.978 INFO Received unsigned block, slot: 5862880, service: block, module: validator_client::block_service:393
> Feb 23 18:56:28.552 INFO Publishing signed block, slot: 5862880, service: block, module: validator_client::block_service:416

The only thing that happens between those two logs is the signing of the block:

0fb58a680d/validator_client/src/block_service.rs (L410-L414)

Helpfully, Stakely noticed this issue without any Lighthouse BNs in the mix, which pointed to a clear issue in the VC.

## TODO

- [x] Further testing on testnet infrastructure.
- [x] Make the attestation signing parallelism configurable.

validator_client/src/http_metrics/metrics.rs

@@ -32,6 +32,7 @@ pub const PROPOSER_DUTIES_HTTP_GET: &str = "proposer_duties_http_get";
 pub const VALIDATOR_ID_HTTP_GET: &str = "validator_id_http_get";
 pub const SUBSCRIPTIONS_HTTP_POST: &str = "subscriptions_http_post";
 pub const UPDATE_PROPOSERS: &str = "update_proposers";
+pub const ATTESTATION_SELECTION_PROOFS: &str = "attestation_selection_proofs";
 pub const SUBSCRIPTIONS: &str = "subscriptions";
 pub const LOCAL_KEYSTORE: &str = "local_keystore";
 pub const WEB3SIGNER: &str = "web3signer";
@@ -172,6 +173,10 @@ lazy_static::lazy_static! {
         "Duration to obtain a signature",
         &["type"]
     );
+    pub static ref BLOCK_SIGNING_TIMES: Result<Histogram> = try_create_histogram(
+        "vc_block_signing_times_seconds",
+        "Duration to obtain a signature for a block",
+    );
 
     pub static ref ATTESTATION_DUTY: Result<IntGaugeVec> = try_create_int_gauge_vec(
         "vc_attestation_duty_slot",