p2p/discover: s/lastPong/bondTime/, update TestUDP_findnode

I forgot to change the check in udp.go when I changed Table.bond to be
based on lastPong instead of node presence in db. Rename lastPong to
bondTime and add hasBond so it's clearer what this DB key is used for
now.

p2p/discover/udp_test.go

@@ -247,12 +247,8 @@ func TestUDP_findnode(t *testing.T) {
 
 	// ensure there's a bond with the test node,
 	// findnode won't be accepted otherwise.
-	test.table.db.updateNode(NewNode(
-		PubkeyID(&test.remotekey.PublicKey),
-		test.remoteaddr.IP,
-		uint16(test.remoteaddr.Port),
-		99,
-	))
+	test.table.db.updateBondTime(PubkeyID(&test.remotekey.PublicKey), time.Now())
+
 	// check that closest neighbors are returned.
 	test.packetIn(nil, findnodePacket, &findnode{Target: testTarget, Expiration: futureExp})
 	expected := test.table.closest(targetHash, bucketSize)