Improved RPC timeout handling (#770)

* Handle slow app level responses for RPC

* Correct errors

* Terminates stream on error

beacon_node/network/src/sync/range_sync/chain.rs

@@ -564,6 +564,9 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
     ) -> ProcessingResult {
         batch.retries += 1;
 
+        // TODO: Handle partially downloaded batches. Update this when building a new batch
+        // processor thread.
+
         if batch.retries > MAX_BATCH_RETRIES {
             // chain is unrecoverable, remove it
             ProcessingResult::RemoveChain