Assert recycler is given last reference to data

This patch likely fixes the sporadic failures in the following tests:

```
test server::tests::validator_exit ... FAILED
test streamer::test::streamer_send_test ... FAILED
test thin_client::tests::test_bad_sig ... FAILED
test drone::tests::test_send_airdrop ... FAILED
test thin_client::tests::test_thin_client ... FAILED
```

src/ncp.rs

@@ -64,10 +64,7 @@ mod tests {
     use std::sync::{Arc, RwLock};
 
     #[test]
-    #[ignore]
     // test that stage will exit when flag is set
-    // TODO: Troubleshoot Docker-based coverage build and re-enabled
-    // this test. It is probably failing due to too many threads.
     fn test_exit() {
         let exit = Arc::new(AtomicBool::new(false));
         let tn = TestNode::new();

src/packet.rs

@@ -166,6 +166,7 @@ impl<T: Default> Recycler<T> {
             .unwrap_or_else(|| Arc::new(RwLock::new(Default::default())))
     }
     pub fn recycle(&self, msgs: Arc<RwLock<T>>) {
+        assert_eq!(Arc::strong_count(&msgs), 1); // Ensure this function holds that last reference.
         let mut gc = self.gc.lock().expect("recycler lock in pub fn recycle");
         gc.push(msgs);
     }

src/streamer.rs

@@ -29,19 +29,18 @@ fn recv_loop(
 ) -> Result<()> {
     loop {
         let msgs = re.allocate();
-        let msgs_ = msgs.clone();
         loop {
-            match msgs.write()
+            let result = msgs.write()
                 .expect("write lock in fn recv_loop")
-                .recv_from(sock)
+                .recv_from(sock);
-            {
+            match result {
                 Ok(()) => {
-                    channel.send(msgs_)?;
+                    channel.send(msgs)?;
                     break;
                 }
                 Err(_) => {
                     if exit.load(Ordering::Relaxed) {
-                        re.recycle(msgs_);
+                        re.recycle(msgs);
                         return Ok(());
                     }
                 }
@@ -760,12 +759,13 @@ mod test {
         let mut msgs = VecDeque::new();
         for i in 0..10 {
             let b = resp_recycler.allocate();
-            let b_ = b.clone();
+            {
                 let mut w = b.write().unwrap();
                 w.data[0] = i as u8;
                 w.meta.size = PACKET_DATA_SIZE;
                 w.meta.set_addr(&addr);
-            msgs.push_back(b_);
+            }
+            msgs.push_back(b);
         }
         s_responder.send(msgs).expect("send");
         let mut num = 0;