Remove sync await points (#3036)

## Issue Addressed

Removes the await points in sync waiting for a processor response for rpc block processing. Built on top of #3029 
This also handles a couple of bugs in the previous code and adds a relatively comprehensive test suite.

beacon_node/network/src/sync/mod.rs

@@ -2,6 +2,7 @@
 //!
 //! Stores the various syncing methods for the beacon chain.
 mod backfill_sync;
+mod block_lookups;
 pub mod manager;
 mod network_context;
 mod peer_sync_info;