les, p2p/discv5: implement in-protocol payment

[zsfelfoldi]

This is a WIP PR for the in-protocol payment logic. Currently implemented features:

• a discv5 extension for protocol negotiation ("talkRequest/talkResponse") and a dummy handler for the "les" domain
• a new clientPool function for calculating the necessary additional amount of token balance in order to fulfill a requested connection/capacity update (the core logic is also deduplicated)

Coming soon:

• new LES/4 and "LESTALK" UDP messages necessary for payment processing and price negotiation
• glue logic for the token sale and payment receiver modules

[rjl493456442]

Please add the function description and relative unit tests.

[rjl493456442]

func (bt *balanceTracker) budget(targetPriority int64, targetCapacity uint64, after time.Duration)  uint64 {
	current := bt.balanceToPriority(bt.balance)
	if targetPriority >= current {
		if bt.negTimeFactor == 0 {
			return 0
		}
		diff :=  time.Duration(float64(targetPriority - current)/bt.negTimeFactor)
		if diff >= after {
			return 0
		}
		after -= diff
	}
	// We have to cover the connection expense by positive balance.
	if targetPriority >= 0 {
		targetPriority = -1
	}
	budget := uint64(float64(^targetPriority)*float64(targetCapacity) + float64(after)*bt.timeFactor)
	if budget >= maxBalance {
		return math.MaxUint64 // target not reachable
	}
	if budget > bt.balance.pos {
		return budget - bt.balance.pos
	}
	return 0
}

[rjl493456442]

Just open a question here. So we can only guarantee the idle connection by the budget. However request can also cost a lot. TBH I think request cost should be much higher than connection cost.

So should we consider the estimated request cost when we calculate the budget? Although we can never have a correct number.

[rjl493456442]

Does it mean servers can only receive the UDP packet and then reply?
So in the following scenarios:

• Client should ask the current token price
• Client should ask the current balance to determine whether they need to buy more tokens for stable connection

[zsfelfoldi]

Superseded by #20517