core, eth: rework snap sync

[Francesco4203]

Reference: ethereum/go-ethereum#24898

[huyngopt1994]

	var onSlot func(paths [][]byte, hexpath []byte, leaf []byte, parent common.Hash) error
	var onSlot func(keys [][]byte, path []byte, leaf []byte, parent common.Hash, parentPath []byte) error

The reason behind why we changed from paths to keys and hexpath to path

keys is a tuple of 32 bytes key. path is the hex format nibbles. Just want to make it less-ambiguous. 

The keys []byte will convert to hex format nibbles via this function

func keybytesToHex(str []byte) []byte {
	l := len(str)*2 + 1
	var nibbles = make([]byte, l)
	for i, b := range str {
		nibbles[i*2] = b / 16
		nibbles[i*2+1] = b % 16
	}
	nibbles[l-1] = 16
	return nibbles
}

[huyngopt1994]

split request logic into node and code, same with SyncResult

[huyngopt1994]

In the previous logic.

err := s.sched.Process(res)

They will forward res to Process function , in the new logic, we need to split into handle either ProcessCode or ProcessNode. Could u explain a little the condition check here for choosing which kind of processing in sched.

[Francesco4203]

Since the requests list is also now divided into nodeReqs and codeReqs, we use ProcessNode for each request taking from nodeReqs and ProcessCode for each request taking from codeReqs

[huyngopt1994]

LGTM, let merge and retest again