Fix stuck backfill when scheduled work queue is at capacity (#5575)

* Fix stuck backfill and add regression test.

* Remove unnecessary `yield_now`

* Merge branch 'unstable' into fix-stuck-backfill

* Revert previous change and add extra comment.

* Merge branch 'unstable' into fix-stuck-backfill

* Update tests to use configured event schedule instead of hard coded values.

* Merge branch 'unstable' of https://github.com/sigp/lighthouse into fix-stuck-backfill

beacon_node/beacon_processor/src/lib.rs

@@ -851,7 +851,7 @@ impl<E: EthSpec> BeaconProcessor<E> {
             ready_work_tx,
             work_reprocessing_rx,
             &self.executor,
-            slot_clock,
+            Arc::new(slot_clock),
             self.log.clone(),
             maximum_gossip_clock_disparity,
         )?;