Implement slasher (#1567)

This is an implementation of a slasher that lives inside the BN and can be enabled via `lighthouse bn --slasher`.

Features included in this PR:

- [x] Detection of attester slashing conditions (double votes, surrounds existing, surrounded by existing)
- [x] Integration into Lighthouse's attestation verification flow
- [x] Detection of proposer slashing conditions
- [x] Extraction of attestations from blocks as they are verified
- [x] Compression of chunks
- [x] Configurable history length
- [x] Pruning of old attestations and blocks
- [x] More tests

Future work:

* Focus on a slice of history separate from the most recent N epochs (e.g. epochs `current - K` to `current - M`)
* Run out-of-process
* Ingest attestations from the chain without a resync

Design notes are here https://hackmd.io/@sproul/HJSEklmPL

beacon_node/src/lib.rs

@@ -17,8 +17,10 @@ use beacon_chain::{
 };
 use clap::ArgMatches;
 use environment::RuntimeContext;
+use slasher::Slasher;
 use slog::{info, warn};
 use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
 use types::EthSpec;
 
 /// A type-alias to the tighten the definition of a production-intended `Client`.
@@ -82,6 +84,16 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
             .http_api_config(client_config.http_api.clone())
             .disk_store(&db_path, &freezer_db_path_res?, store_config)?;
 
+        let builder = if let Some(slasher_config) = client_config.slasher.clone() {
+            let slasher = Arc::new(
+                Slasher::open(slasher_config, log.new(slog::o!("service" => "slasher")))
+                    .map_err(|e| format!("Slasher open error: {:?}", e))?,
+            );
+            builder.slasher(slasher)
+        } else {
+            builder
+        };
+
         let builder = builder
             .beacon_chain_builder(client_genesis, client_config_1)
             .await?;