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();

beacon_node/src/cli.rs

@@ -515,12 +515,20 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("slasher-max-db-size")
                 .long("slasher-max-db-size")
                 .help(
-                    "Maximum size of the LMDB database used by the slasher."
+                    "Maximum size of the MDBX database used by the slasher."
                 )
                 .value_name("GIGABYTES")
                 .requires("slasher")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("slasher-att-cache-size")
+                .long("slasher-att-cache-size")
+                .help("Set the maximum number of attestation roots for the slasher to cache")
+                .value_name("COUNT")
+                .requires("slasher")
+                .takes_value(true)
+        )
         .arg(
             Arg::with_name("slasher-chunk-size")
                 .long("slasher-chunk-size")

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;
         }