Rate limiting backfill sync (#3936)

## Issue Addressed

#3212 

## Proposed Changes

- Introduce a new `rate_limiting_backfill_queue` - any new inbound backfill work events gets immediately sent to this FIFO queue **without any processing**
- Spawn a `backfill_scheduler` routine that pops a backfill event from the FIFO queue at specified intervals (currently halfway through a slot, or at 6s after slot start for 12s slots) and sends the event to `BeaconProcessor` via a `scheduled_backfill_work_tx` channel
- This channel gets polled last in the `InboundEvents`, and work event received is  wrapped in a `InboundEvent::ScheduledBackfillWork` enum variant, which gets processed immediately or queued by the `BeaconProcessor` (existing logic applies from here)

Diagram comparing backfill processing with / without rate-limiting: 
https://github.com/sigp/lighthouse/issues/3212#issuecomment-1386249922

See this comment for @paulhauner's  explanation and solution: https://github.com/sigp/lighthouse/issues/3212#issuecomment-1384674956

## Additional Info

I've compared this branch (with backfill processing rate limited to to 1 and 3 batches per slot) against the latest stable version. The CPU usage during backfill sync is reduced by ~5% - 20%, more details on this page:

https://hackmd.io/@jimmygchen/SJuVpJL3j

The above testing is done on Goerli (as I don't currently have hardware for Mainnet), I'm guessing the differences are likely to be bigger on mainnet due to block size.

### TODOs

- [x] Experiment with processing multiple batches per slot. (need to think about how to do this for different slot durations)
- [x] Add option to disable rate-limiting, enabed by default.
- [x] (No longer required now we're reusing the reprocessing queue) Complete the `backfill_scheduler` task when backfill sync is completed or not required

lighthouse/tests/beacon_node.rs

@@ -1052,6 +1052,19 @@ fn disable_upnp_flag() {
         .with_config(|config| assert!(!config.network.upnp_enabled));
 }
 #[test]
+fn disable_backfill_rate_limiting_flag() {
+    CommandLineTest::new()
+        .flag("disable-backfill-rate-limiting", None)
+        .run_with_zero_port()
+        .with_config(|config| assert!(!config.chain.enable_backfill_rate_limiting));
+}
+#[test]
+fn default_backfill_rate_limiting_flag() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.chain.enable_backfill_rate_limiting));
+}
+#[test]
 fn default_boot_nodes() {
     let mainnet = vec![
     // Lighthouse Team (Sigma Prime)