Refactor block_processing

- Add the rayon library for parallelization
- Move from std::sync::Rwlock to rayon::RwLock
- Add `state` field to BeaconChain
- Fix major bug in attestation validator where justified slot was
incorrectly looked up.

beacon_node/beacon_chain/test_harness/tests/chain.rs

@@ -24,7 +24,7 @@ fn it_can_build_on_genesis_block() {
 fn it_can_produce_past_first_epoch_boundary() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
 
-    let validator_count = 100;
+    let validator_count = 128 * 1024;
 
     debug!("Starting harness build...");