les: rework clientpool (#20077)

* les: rework clientpool

les/request_test.go

@@ -81,8 +81,15 @@ func testAccess(t *testing.T, protocol int, fn accessTestFn) {
 	// Assemble the test environment
 	server, client, tearDown := newClientServerEnv(t, 4, protocol, nil, nil, 0, false, true)
 	defer tearDown()
+
 	client.handler.synchronise(client.peer.peer)
 
+	// Ensure the client has synced all necessary data.
+	clientHead := client.handler.backend.blockchain.CurrentHeader()
+	if clientHead.Number.Uint64() != 4 {
+		t.Fatalf("Failed to sync the chain with server, head: %v", clientHead.Number.Uint64())
+	}
+
 	test := func(expFail uint64) {
 		for i := uint64(0); i <= server.handler.blockchain.CurrentHeader().Number.Uint64(); i++ {
 			bhash := rawdb.ReadCanonicalHash(server.db, i)