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/utils.rs

@@ -1,20 +1,5 @@
 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()),
-        }
-    }
-}
-
 /// Transform a transaction that would fail with a `MapFull` error into an optional result.
 pub trait TxnMapFull<T, E> {
     fn allow_map_full(self) -> Result<Option<T>, E>;
@@ -24,7 +9,7 @@ impl<T> TxnMapFull<T, Error> for Result<T, Error> {
     fn allow_map_full(self) -> Result<Option<T>, Error> {
         match self {
             Ok(x) => Ok(Some(x)),
-            Err(Error::DatabaseError(lmdb::Error::MapFull)) => Ok(None),
+            Err(Error::DatabaseError(mdbx::Error::MapFull)) => Ok(None),
             Err(e) => Err(e),
         }
     }