stanzastream: actually count stanzas in stream management

Looks like a horrible oversight. And this will need tests, but I'm short
on time right now.

Fixes #170.

tokio-xmpp/src/stanzastream/connected.rs

@@ -543,6 +543,9 @@ impl ConnectedState {
                 match item {
                     // Easy case, we got some data.
                     Ok(XmppStreamElement::Stanza(data)) => {
+                        if let Some(sm_state) = sm_state.as_mut() {
+                            sm_state.received();
+                        }
                         Poll::Ready(Some(ConnectedEvent::Worker(WorkerEvent::Stanza(data))))
                     }
 

tokio-xmpp/src/stanzastream/stream_management.rs

@@ -214,6 +214,11 @@ impl SmState {
         self.inbound_ctr
     }
 
+    /// Increase the inbound counter.
+    pub fn received(&mut self) {
+        self.inbound_ctr = self.inbound_ctr.wrapping_add(1);
+    }
+
     /// Get the info necessary for resumption.
     ///
     /// Returns the stream ID and the current inbound counter if resumption is