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

common/lighthouse_metrics/src/lib.rs

@@ -143,7 +143,7 @@ pub fn try_create_float_gauge_vec(
     Ok(counter_vec)
 }
 
-/// Attempts to create a `IntGaugeVec`, returning `Err` if the registry does not accept the gauge
+/// Attempts to create a `IntCounterVec`, returning `Err` if the registry does not accept the gauge
 /// (potentially due to naming conflict).
 pub fn try_create_int_counter_vec(
     name: &str,
@@ -221,6 +221,12 @@ pub fn inc_counter_vec(int_counter_vec: &Result<IntCounterVec>, name: &[&str]) {
     }
 }
 
+pub fn inc_counter_vec_by(int_counter_vec: &Result<IntCounterVec>, name: &[&str], amount: i64) {
+    if let Some(counter) = get_int_counter(int_counter_vec, name) {
+        counter.inc_by(amount);
+    }
+}
+
 /// If `histogram_vec.is_ok()`, returns a histogram with the given `name`.
 pub fn get_histogram(histogram_vec: &Result<HistogramVec>, name: &[&str]) -> Option<Histogram> {
     if let Ok(histogram_vec) = histogram_vec {