les: implement ultralight client (#16904)

For more information about this light client mode, read
https://hackmd.io/s/HJy7jjZpm

les/ulc.go

@@ -0,0 +1,39 @@
+package les
+
+import (
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/eth"
+	"github.com/ethereum/go-ethereum/p2p/enode"
+)
+
+type ulc struct {
+	trustedKeys        map[string]struct{}
+	minTrustedFraction int
+}
+
+func newULC(ulcConfig *eth.ULCConfig) *ulc {
+	if ulcConfig == nil {
+		return nil
+	}
+
+	m := make(map[string]struct{}, len(ulcConfig.TrustedServers))
+	for _, id := range ulcConfig.TrustedServers {
+		node, err := enode.ParseV4(id)
+		if err != nil {
+			fmt.Println("node:", id, " err:", err)
+			continue
+		}
+		m[node.ID().String()] = struct{}{}
+	}
+
+	return &ulc{m, ulcConfig.MinTrustedFraction}
+}
+
+func (u *ulc) isTrusted(p enode.ID) bool {
+	if u.trustedKeys == nil {
+		return false
+	}
+	_, ok := u.trustedKeys[p.String()]
+	return ok
+}