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/wait_for_activation_test.go

@@ -194,7 +194,7 @@ func TestWaitForActivation_AccountsChanged(t *testing.T) {
 		v := validator{
 			validatorClient:  validatorClient,
 			km:               km,
-			genesisTime:      1,
+			genesisTime:      time.Unix(1, 0),
 			chainClient:      chainClient,
 			prysmChainClient: prysmChainClient,
 			pubkeyToStatus:   make(map[[48]byte]*validatorStatus),