Fix syncing bugs by recursively attempting to process parents in the … (#429)

* Fix syncing bugs by recursively attempting to process parents in the import queue, change BlockRootsIterator

* Swap from crossbeam channel to tokio mpsc

* Recursion fix

* Remove exess block processing

* Fix network lag, correct attestation topic

* Correct network poll logic

* Overhaul of SimpleSync and modify BlockRootsIterator to return start_slot

* Fix bug in tests relating to StateRootsIterator

* Remove old, commented-out heartbeat code.

* Tidy docs on import queue enum

* Change source logging msg in simple sync

* Rename function parameter in simple sync

* Use `BestBlockRootsIterator` in `reduced_tree`

* Update comments for `BestBlockRootsIterator`

* Fix duplicate dep in cargo.toml

beacon_node/eth2-libp2p/src/service.rs

@@ -109,8 +109,6 @@ impl Stream for Service {
 
     fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
         loop {
-            // TODO: Currently only gossipsub events passed here.
-            // Build a type for more generic events
             match self.swarm.poll() {
                 //Behaviour events
                 Ok(Async::Ready(Some(event))) => match event {