Move over EvTracker to a normal event sender

I considered removing it from the context by default, but the
migration test really wants to have the tracker initialised from the
very first event and not after the context is initialised.  It is
easier for now to leave it hardcoded instead of adding an API to
explicitly require enabling it via the builder.

src/dc_receive_imf.rs

@@ -4684,14 +4684,18 @@ Second thread."#;
         let chat = chat::Chat::load_from_db(&t, msg.chat_id).await?;
         assert!(chat.is_contact_request());
 
-        let duration = std::time::Duration::from_secs(1);
         loop {
-            let event = async_std::future::timeout(duration, t.evtracker.recv()).await??;
-
-            if let EventType::IncomingMsg { chat_id, msg_id } = &event {
-                assert_eq!(msg.chat_id, *chat_id);
-                assert_eq!(msg.id, *msg_id);
-                return Ok(());
+            let event = t
+                .evtracker
+                .get_matching(|evt| matches!(evt, EventType::IncomingMsg { .. }))
+                .await;
+            match event {
+                EventType::IncomingMsg { chat_id, msg_id } => {
+                    assert_eq!(msg.chat_id, chat_id);
+                    assert_eq!(msg.id, msg_id);
+                    return Ok(());
+                }
+                _ => unreachable!(),
             }
         }
     }