les: code refactoring (#14416)

This commit does various code refactorings:

- generalizes and moves the request retrieval/timeout/resend logic out of LesOdr
  (will be used by a subsequent PR)
- reworks the peer management logic so that all services can register with
  peerSet to get notified about added/dropped peers (also gets rid of the ugly
  getAllPeers callback in requestDistributor)
- moves peerSet, LesOdr, requestDistributor and retrieveManager initialization
  out of ProtocolManager because I believe they do not really belong there and the
  whole init process was ugly and ad-hoc

les/distributor_test.go

@@ -122,20 +122,14 @@ func testRequestDistributor(t *testing.T, resend bool) {
 	stop := make(chan struct{})
 	defer close(stop)
 
+	dist := newRequestDistributor(nil, stop)
 	var peers [testDistPeerCount]*testDistPeer
 	for i, _ := range peers {
 		peers[i] = &testDistPeer{}
 		go peers[i].worker(t, !resend, stop)
+		dist.registerTestPeer(peers[i])
 	}
 
-	dist := newRequestDistributor(func() map[distPeer]struct{} {
-		m := make(map[distPeer]struct{})
-		for _, peer := range peers {
-			m[peer] = struct{}{}
-		}
-		return m
-	}, stop)
-
 	var wg sync.WaitGroup
 
 	for i := 1; i <= testDistReqCount; i++ {