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/src/config.rs

@@ -510,6 +510,12 @@ pub fn get_config<E: EthSpec>(
             slasher_config.max_db_size_mbs = max_db_size_gbs * 1024;
         }
 
+        if let Some(attestation_cache_size) =
+            clap_utils::parse_optional(cli_args, "slasher-att-cache-size")?
+        {
+            slasher_config.attestation_root_cache_size = attestation_cache_size;
+        }
+
         if let Some(chunk_size) = clap_utils::parse_optional(cli_args, "slasher-chunk-size")? {
             slasher_config.chunk_size = chunk_size;
         }