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>

tools/exploredb/main.go

@@ -326,7 +326,7 @@ func printStates(stateC <-chan *modifiedState, doneC chan<- bool) {
 		log.Infof("---- row = %04d, slot = %8d, epoch = %8d, key = %s ----", mst.rowCount, st.Slot(), st.Slot()/params.BeaconConfig().SlotsPerEpoch, hexutils.BytesToHex(mst.key))
 		log.Infof("key                           : %s", hexutils.BytesToHex(mst.key))
 		log.Infof("value                         : compressed size = %s", humanize.Bytes(mst.valueSize))
-		t := time.Unix(int64(st.GenesisTime()), 0) // lint:ignore uintcast -- Genesis time will not exceed int64 in your lifetime.
+		t := st.GenesisTime()
 		log.Infof("genesis_time                  : %s", t.Format(time.UnixDate))
 		log.Infof("genesis_validators_root       : %s", hexutils.BytesToHex(st.GenesisValidatorsRoot()))
 		log.Infof("slot                          : %d", st.Slot())