Add first changes to syncing logic

- Adds testing framework
- Breaks out new `NetworkContext` object

beacon_node/network/src/lib.rs

@@ -1,8 +1,8 @@
 /// This crate provides the network server for Lighthouse.
 pub mod beacon_chain;
 pub mod error;
-mod message_handler;
-mod service;
+pub mod message_handler;
+pub mod service;
 pub mod sync;
 
 pub use libp2p::NetworkConfig;