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

beacon_node/src/cli.rs

@@ -261,6 +261,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             .min_values(0)
             .hidden(true)
         )
+        .arg(
+            Arg::with_name("disable-backfill-rate-limiting")
+                .long("disable-backfill-rate-limiting")
+                .help("Disable the backfill sync rate-limiting. This allow users to just sync the entire chain as fast \
+                    as possible, however it can result in resource contention which degrades staking performance. Stakers \
+                    should generally choose to avoid this flag since backfill sync is not required for staking.")
+                .takes_value(false),
+        )
         /* REST API related arguments */
         .arg(
             Arg::with_name("http")

beacon_node/src/config.rs

@@ -759,6 +759,10 @@ pub fn get_config<E: EthSpec>(
         client_config.always_prefer_builder_payload = true;
     }
 
+    // Backfill sync rate-limiting
+    client_config.chain.enable_backfill_rate_limiting =
+        !cli_args.is_present("disable-backfill-rate-limiting");
+
     Ok(client_config)
 }