Fix durationToNextSlot() and durationToNextEpoch() to work not only after Genesis, but also before Genesis.

Change VC pre-genesis behavior, add runPreGenesisWaitingLoop() and runGenesisWaitingLoop().
Add checkedWaitForSlot() and checkedWaitForNextSlot() to strictly check current time and print warnings.
Fix VC main loop to use checkedWaitForNextSlot().
Fix attestation_service to run attestations processing only until the end of the duty slot.
Change attestation_service main loop to use checkedWaitForNextSlot().
Change block_service to properly cancel all the pending proposer tasks.
Use checkedWaitForSlot to wait for block proposal.
Fix block_service waitForBlockPublished() to be compatible with BN.
Fix sync_committee_service to avoid asyncSpawn.
Fix sync_committee_service to run only until the end of the duty slot.
Fix sync_committee_service to use checkedWaitForNextSlot().

beacon_chain/beacon_clock.nim

@@ -16,6 +16,10 @@ from times import Time, getTime, fromUnix, `<`, `-`, inNanoseconds
 
 export chronos.Duration, Moment, now
 
+const
+  NANOSECONDS_PER_SLOT* = SECONDS_PER_SLOT * 1_000_000_000'u64
+  NANOSECONDS_PER_EPOCH* = NANOSECONDS_PER_SLOT * SLOTS_PER_EPOCH
+
 type
   BeaconClock* = object
     ## The beacon clock represents time as it passes on a beacon chain. Beacon

beacon_chain/validator_client/common.nim

@@ -936,6 +936,7 @@ proc checkedWaitForSlot*(vc: ValidatorClientRef, destinationSlot: Slot,
     start_slot = shortLog(currentSlot)
     dest_slot = shortLog(destinationSlot)
     time_to_slot = shortLog(timeToSlot)
+    offset = shortLog(chronosOffset)
 
   while true:
     await sleepAsync(timeToSlot)