Block backfilling (#12968)

* backfill service

* fix bug where origin state is never unlocked

* support mvslice states

* use renamed interface

* refactor db code to skip block cache for backfill

* lint

* add test for verifier.verify

* enable service in service init test

* cancellation cleanup

* adding nil checks to configset juggling

* assume blocks are available by default

As long as we're sure the AvailableBlocker is initialized correctly
during node startup, defaulting to assuming we aren't in a checkpoint
sync simplifies things greatly for tests.

* block saving path refactor and bugfix

* fix fillback test

* fix BackfillStatus init tests

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>

time/slots/slottime.go

@@ -98,6 +98,16 @@ func EpochStart(epoch primitives.Epoch) (primitives.Slot, error) {
 	return slot, nil
 }
 
+// UnsafeEpochStart is a version of EpochStart that panics if there is an overflow. It can be safely used by code
+// that first guarantees epoch <= MaxSafeEpoch.
+func UnsafeEpochStart(epoch primitives.Epoch) primitives.Slot {
+	es, err := EpochStart(epoch)
+	if err != nil {
+		panic(err)
+	}
+	return es
+}
+
 // EpochEnd returns the last slot number of the
 // current epoch.
 func EpochEnd(epoch primitives.Epoch) (primitives.Slot, error) {
@@ -271,3 +281,8 @@ func WithinVotingWindow(genesisTime uint64, slot primitives.Slot) bool {
 	votingWindow := params.BeaconConfig().SecondsPerSlot / params.BeaconConfig().IntervalsPerSlot
 	return time.Since(StartTime(genesisTime, slot)) < time.Duration(votingWindow)*time.Second
 }
+
+// MaxSafeEpoch gives the largest epoch value that can be safely converted to a slot.
+func MaxSafeEpoch() primitives.Epoch {
+	return primitives.Epoch(math.MaxUint64 / uint64(params.BeaconConfig().SlotsPerEpoch))
+}