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/store/src/metrics.rs

@@ -1,6 +1,6 @@
 pub use lighthouse_metrics::{set_gauge, try_create_int_gauge, *};
 
-use std::fs;
+use directory::size_of_dir;
 use std::path::Path;
 
 lazy_static! {
@@ -134,17 +134,3 @@ pub fn scrape_for_metrics(db_path: &Path, freezer_db_path: &Path) {
     let freezer_db_size = size_of_dir(freezer_db_path);
     set_gauge(&FREEZER_DB_SIZE, freezer_db_size as i64);
 }
-
-fn size_of_dir(path: &Path) -> u64 {
-    if let Ok(iter) = fs::read_dir(path) {
-        iter.filter_map(std::result::Result::ok)
-            .map(size_of_dir_entry)
-            .sum()
-    } else {
-        0
-    }
-}
-
-fn size_of_dir_entry(dir: fs::DirEntry) -> u64 {
-    dir.metadata().map(|m| m.len()).unwrap_or(0)
-}