Optimise slasher DB layout and switch to MDBX (#2776)

## Issue Addressed

Closes #2286
Closes #2538
Closes #2342

## Proposed Changes

Part II of major slasher optimisations after #2767

These changes will be backwards-incompatible due to the move to MDBX (and the schema change) 😱 

* [x] Shrink attester keys from 16 bytes to 7 bytes.
* [x] Shrink attester records from 64 bytes to 6 bytes.
* [x] Separate `DiskConfig` from regular `Config`.
* [x] Add configuration for the LRU cache size.
* [x] Add a "migration" that deletes any legacy LMDB database.

slasher/src/attester_record.rs

@@ -1,17 +1,53 @@
+use crate::{database::IndexedAttestationId, Error};
 use ssz_derive::{Decode, Encode};
-use std::sync::Arc;
+use std::borrow::Cow;
+use std::sync::{
+    atomic::{AtomicU64, Ordering},
+    Arc,
+};
 use tree_hash::TreeHash as _;
 use tree_hash_derive::TreeHash;
 use types::{AggregateSignature, EthSpec, Hash256, IndexedAttestation, VariableList};
 
-#[derive(Debug, Clone, Copy, Encode, Decode)]
+#[derive(Debug, Clone, Copy)]
 pub struct AttesterRecord {
-    /// The hash of the attestation data, for checking double-voting.
+    /// The hash of the attestation data, for de-duplication.
     pub attestation_data_hash: Hash256,
     /// The hash of the indexed attestation, so it can be loaded.
     pub indexed_attestation_hash: Hash256,
 }
 
+#[derive(Debug, Clone, Copy)]
+pub struct CompactAttesterRecord {
+    /// The ID of the `IndexedAttestation` signed by this validator.
+    pub indexed_attestation_id: IndexedAttestationId,
+}
+
+impl CompactAttesterRecord {
+    pub fn new(indexed_attestation_id: IndexedAttestationId) -> Self {
+        Self {
+            indexed_attestation_id,
+        }
+    }
+
+    pub fn null() -> Self {
+        Self::new(IndexedAttestationId::null())
+    }
+
+    pub fn parse(bytes: Cow<[u8]>) -> Result<Self, Error> {
+        let id = IndexedAttestationId::parse(bytes)?;
+        Ok(Self::new(IndexedAttestationId::new(id)))
+    }
+
+    pub fn is_null(&self) -> bool {
+        self.indexed_attestation_id.is_null()
+    }
+
+    pub fn as_bytes(&self) -> &[u8] {
+        self.indexed_attestation_id.as_ref()
+    }
+}
+
 /// Bundling of an `IndexedAttestation` with an `AttesterRecord`.
 ///
 /// This struct gets `Arc`d and passed around between each stage of queueing and processing.
@@ -19,11 +55,26 @@ pub struct AttesterRecord {
 pub struct IndexedAttesterRecord<E: EthSpec> {
     pub indexed: IndexedAttestation<E>,
     pub record: AttesterRecord,
+    pub indexed_attestation_id: AtomicU64,
 }
 
 impl<E: EthSpec> IndexedAttesterRecord<E> {
     pub fn new(indexed: IndexedAttestation<E>, record: AttesterRecord) -> Arc<Self> {
-        Arc::new(IndexedAttesterRecord { indexed, record })
+        Arc::new(IndexedAttesterRecord {
+            indexed,
+            record,
+            indexed_attestation_id: AtomicU64::new(0),
+        })
+    }
+
+    pub fn set_id(&self, id: u64) {
+        self.indexed_attestation_id
+            .compare_exchange(0, id, Ordering::Relaxed, Ordering::Relaxed)
+            .expect("IDs should only be initialized once");
+    }
+
+    pub fn get_id(&self) -> u64 {
+        self.indexed_attestation_id.load(Ordering::Relaxed)
     }
 }