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/rpc/src/beacon_block.rs

@@ -1,5 +1,4 @@
 use beacon_chain::{BeaconChain, BeaconChainTypes, BlockProcessingOutcome};
-use crossbeam_channel;
 use eth2_libp2p::BEACON_PUBSUB_TOPIC;
 use eth2_libp2p::{PubsubMessage, TopicBuilder};
 use futures::Future;
@@ -14,12 +13,13 @@ use slog::Logger;
 use slog::{error, info, trace, warn};
 use ssz::{ssz_encode, Decode};
 use std::sync::Arc;
+use tokio::sync::mpsc;
 use types::{BeaconBlock, Signature, Slot};
 
 #[derive(Clone)]
 pub struct BeaconBlockServiceInstance<T: BeaconChainTypes> {
     pub chain: Arc<BeaconChain<T>>,
-    pub network_chan: crossbeam_channel::Sender<NetworkMessage>,
+    pub network_chan: mpsc::UnboundedSender<NetworkMessage>,
     pub log: Logger,
 }
 
@@ -111,7 +111,7 @@ impl<T: BeaconChainTypes> BeaconBlockService for BeaconBlockServiceInstance<T> {
 
                             // Publish the block to the p2p network via gossipsub.
                             self.network_chan
-                                .send(NetworkMessage::Publish {
+                                .try_send(NetworkMessage::Publish {
                                     topics: vec![topic],
                                     message: Box::new(message),
                                 })