Process gossip blocks on the GossipProcessor (#1523)

## Issue Addressed

NA

## Proposed Changes

Moves beacon block processing over to the newly-added `GossipProcessor`. This moves the task off the core executor onto the blocking one.

## Additional Info

- With this PR, gossip blocks are being ignored during sync.

beacon_node/network/src/sync/mod.rs

@@ -1,14 +1,14 @@
 //! Syncing for lighthouse.
 //!
 //! Stores the various syncing methods for the beacon chain.
-mod block_processor;
 pub mod manager;
 mod network_context;
 mod peer_sync_info;
 mod range_sync;
 
-pub use manager::SyncMessage;
+pub use manager::{BatchProcessResult, SyncMessage};
 pub use peer_sync_info::PeerSyncInfo;
+pub use range_sync::{BatchId, ChainId};
 
 /// Type of id of rpc requests sent by sync
 pub type RequestId = usize;