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.

beacon_node/beacon_chain/tests/block_verification.rs

@@ -830,11 +830,7 @@ fn block_gossip_verification() {
 fn verify_block_for_gossip_slashing_detection() {
     let slasher_dir = tempdir().unwrap();
     let slasher = Arc::new(
-        Slasher::open(
-            SlasherConfig::new(slasher_dir.path().into()).for_testing(),
-            test_logger(),
-        )
-        .unwrap(),
+        Slasher::open(SlasherConfig::new(slasher_dir.path().into()), test_logger()).unwrap(),
     );
 
     let inner_slasher = slasher.clone();