add delay before broadcasting lc objects

[Inspector-Butters]

What type of PR is this?
Bug fix

What does this PR do? Why is it needed?
As per spec, light client p2p messages should be broadcasted with a delay of ~4 seconds after the slot start time. This PR adds that necessary delay.

It also adds the BASIS_POINTS values and logic to calculate component durations based on basis points.

[rkapka]

There is also code in /beacon-chain/sync/validate_light_client.go that deals with timing. It's based on the old spec (

prysm/beacon-chain/sync/validate_light_client.go

Lines 50 to 65 in cc2565a

	// [IGNORE] The optimistic_update is received after the block at signature_slot was given enough time
	// to propagate through the network -- i.e. validate that one-third of optimistic_update.signature_slot
	// has transpired (SECONDS_PER_SLOT / INTERVALS_PER_SLOT seconds after the start of the slot,
	// with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance)
	slotStart, err := slots.StartTime(s.cfg.clock.GenesisTime(), newUpdate.SignatureSlot())
	if err != nil {
	log.WithError(err).Debug("Peer sent a slot that would overflow slot start time")
	return pubsub.ValidationReject, nil
	}
	earliestValidTime := slotStart.
	Add(time.Second * time.Duration(params.BeaconConfig().SecondsPerSlot/params.BeaconConfig().IntervalsPerSlot)).
	Add(-params.BeaconConfig().MaximumGossipClockDisparityDuration())
	if s.cfg.clock.Now().Before(earliestValidTime) {
	log.Debug("Newly received light client optimistic update ignored. not enough time passed for block to propagate")
	return pubsub.ValidationIgnore, nil
	}

and

prysm/beacon-chain/sync/validate_light_client.go

Lines 117 to 132 in cc2565a

	// [IGNORE] The optimistic_update is received after the block at signature_slot was given enough time
	// to propagate through the network -- i.e. validate that one-third of optimistic_update.signature_slot
	// has transpired (SECONDS_PER_SLOT / INTERVALS_PER_SLOT seconds after the start of the slot,
	// with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance)
	slotStart, err := slots.StartTime(s.cfg.clock.GenesisTime(), newUpdate.SignatureSlot())
	if err != nil {
	log.WithError(err).Debug("Peer sent a slot that would overflow slot start time")
	return pubsub.ValidationReject, nil
	}
	earliestValidTime := slotStart.
	Add(time.Second * time.Duration(params.BeaconConfig().SecondsPerSlot/params.BeaconConfig().IntervalsPerSlot)).
	Add(-params.BeaconConfig().MaximumGossipClockDisparityDuration())
	if s.cfg.clock.Now().Before(earliestValidTime) {
	log.Debug("Newly received light client finality update ignored. not enough time passed for block to propagate")
	return pubsub.ValidationIgnore, nil
	}

). Can you update these places to use the new BP construct too?

[rkapka]

What if we changed this to 1? Will the test still pass?

[Inspector-Butters]

yes the test will pass. but we want to be able to test that enough time has passed. if you make it 1, you might as well make it zero.