p2p/discover: split out discv4 code

This change restructures the internals of p2p/discover to make room for
the discv5 code which will soon be added to this package.

- packet type names now have a "V4" suffix.
- ListenUDP returns *UDPv4 instead of *Table. This technically breaks
  the API but the only caller in go-ethereum is package p2p, which uses
  a compatible interface and doesn't need changes.
- The internal transport interface is changed to make Table reusable for v5.
- The 'lookup' code moves from table to transport. This required
  updating the lookup unit test to use udpTest instead of a custom transport.

p2p/discover/common.go

@@ -0,0 +1,57 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package discover
+
+import (
+	"crypto/ecdsa"
+	"net"
+
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/netutil"
+)
+
+type UDPConn interface {
+	ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error)
+	WriteToUDP(b []byte, addr *net.UDPAddr) (n int, err error)
+	Close() error
+	LocalAddr() net.Addr
+}
+
+// Config holds Table-related settings.
+type Config struct {
+	// These settings are required and configure the UDP listener:
+	PrivateKey *ecdsa.PrivateKey
+
+	// These settings are optional:
+	NetRestrict *netutil.Netlist  // network whitelist
+	Bootnodes   []*enode.Node     // list of bootstrap nodes
+	Unhandled   chan<- ReadPacket // unhandled packets are sent on this channel
+	Log         log.Logger        // if set, log messages go here
+}
+
+// ListenUDP starts listening for discovery packets on the given UDP socket.
+func ListenUDP(c UDPConn, ln *enode.LocalNode, cfg Config) (*UDPv4, error) {
+	return ListenV4(c, ln, cfg)
+}
+
+// ReadPacket is a packet that couldn't be handled. Those packets are sent to the unhandled
+// channel if configured.
+type ReadPacket struct {
+	Data []byte
+	Addr *net.UDPAddr
+}