Use time.Time instead of uint64 for genesis time (#15419)

* Convert genesis times from seconds to time.Time

* Fixing failed forkchoice tests in a new commit so it doesn't get worse

Fixing failed spectest tests in a new commit so it doesn't get worse

Fixing forkchoice tests, then spectests

* Fixing forkchoice tests, then spectests. Now asking for help...

* Fix TestForkChoice_GetProposerHead

* Fix broken build

* Resolve TODO(preston) items

* Changelog fragment

* Resolve TODO(preston) items again

* Resolve lint issues

* Use consistant field names for sinceSlotStart (no spaces)

* Manu's feedback

* Renamed StartTime -> UnsafeStartTime, marked as deprecated because it doesn't handle overflow scenarios.
Renamed SlotTime -> StartTime
Renamed SlotAt -> At
Handled the error in cases where StartTime was used.

@james-prysm feedback

* Revert beacon-chain/blockchain/receive_block_test.go from 1b7844de

* Fixing issues after rebase

* Accepted suggestions from @potuz

* Remove CanonicalHeadSlot from merge conflicts

---------

Co-authored-by: potuz <potuz@prysmaticlabs.com>

validator/client/aggregate.go

@@ -217,7 +217,11 @@ func (v *validator) waitToSlotTwoThirds(ctx context.Context, slot primitives.Slo
 	twoThird := oneThird + oneThird
 	delay := twoThird
 
-	startTime := slots.StartTime(v.genesisTime, slot)
+	startTime, err := slots.StartTime(v.genesisTime, slot)
+	if err != nil {
+		log.WithError(err).WithField("slot", slot).Error("Slot overflows, unable to wait for slot two thirds!")
+		return
+	}
 	finalTime := startTime.Add(delay)
 	wait := prysmTime.Until(finalTime)
 	if wait <= 0 {