network/config: redesign ping timeout handling a bit

1) Make timeout a timeout, don't do magic ping counts.
2) Drop additional timer from the main peer's protocol loop, create it
   dynamically and make it disconnect the peer.
3) Don't expose the ping counter to the outside, handle more logic inside the
   Peer.

Relates to #430.

pkg/network/helper_test.go

@@ -160,7 +160,7 @@ type localPeer struct {
 	handshaked     bool
 	t              *testing.T
 	messageHandler func(t *testing.T, msg *Message)
-	pingSent int
+	pingSent       int
 }
 
 func newLocalPeer(t *testing.T) *localPeer {
@@ -206,9 +206,6 @@ func (p *localPeer) Version() *payload.Version {
 func (p *localPeer) LastBlockIndex() uint32 {
 	return p.lastBlockIndex
 }
-func (p *localPeer) UpdateLastBlockIndex(newIndex uint32) {
-	p.lastBlockIndex = newIndex
-}
 func (p *localPeer) HandleVersion(v *payload.Version) error {
 	p.version = v
 	return nil
@@ -225,11 +222,14 @@ func (p *localPeer) HandleVersionAck() error {
 	p.handshaked = true
 	return nil
 }
-func (p *localPeer) GetPingSent() int {
-	return p.pingSent
+func (p *localPeer) SendPing() error {
+	p.pingSent++
+	return nil
 }
-func (p *localPeer) UpdatePingSent(newValue int) {
-	p.pingSent = newValue
+func (p *localPeer) HandlePong(pong *payload.Ping) error {
+	p.lastBlockIndex = pong.LastBlockIndex
+	p.pingSent--
+	return nil
 }
 
 func (p *localPeer) Handshaked() bool {