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

slasher/src/utils.rs

@@ -0,0 +1,16 @@
+use crate::Error;
+
+/// Mix-in trait for loading values from LMDB that may or may not exist.
+pub trait TxnOptional<T, E> {
+    fn optional(self) -> Result<Option<T>, E>;
+}
+
+impl<T> TxnOptional<T, Error> for Result<T, lmdb::Error> {
+    fn optional(self) -> Result<Option<T>, Error> {
+        match self {
+            Ok(x) => Ok(Some(x)),
+            Err(lmdb::Error::NotFound) => Ok(None),
+            Err(e) => Err(e.into()),
+        }
+    }
+}