Modularise slasher backend (#3443)

## Proposed Changes

Enable multiple database backends for the slasher, either MDBX (default) or LMDB. The backend can be selected using `--slasher-backend={lmdb,mdbx}`.

## Additional Info

In order to abstract over the two library's different handling of database lifetimes I've used `Box::leak` to give the `Environment` type a `'static` lifetime. This was the only way I could think of using 100% safe code to construct a self-referential struct `SlasherDB`, where the `OpenDatabases` refers to the `Environment`. I think this is OK, as the `Environment` is expected to live for the life of the program, and both database engines leave the database in a consistent state after each write. The memory claimed for memory-mapping will be freed by the OS and appropriately flushed regardless of whether the `Environment` is actually dropped.

We are depending on two `sigp` forks of `libmdbx-rs` and `lmdb-rs`, to give us greater control over MDBX OS support and LMDB's version.

lighthouse/tests/beacon_node.rs

@@ -1288,6 +1288,32 @@ fn slasher_broadcast_flag() {
             assert!(slasher_config.broadcast);
         });
 }
+
+#[test]
+fn slasher_backend_default() {
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .run_with_zero_port()
+        .with_config(|config| {
+            let slasher_config = config.slasher.as_ref().unwrap();
+            assert_eq!(slasher_config.backend, slasher::DatabaseBackend::Mdbx);
+        });
+}
+
+#[test]
+fn slasher_backend_override_to_default() {
+    // Hard to test this flag because all but one backend is disabled by default and the backend
+    // called "disabled" results in a panic.
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .flag("slasher-backend", Some("mdbx"))
+        .run_with_zero_port()
+        .with_config(|config| {
+            let slasher_config = config.slasher.as_ref().unwrap();
+            assert_eq!(slasher_config.backend, slasher::DatabaseBackend::Mdbx);
+        });
+}
+
 #[test]
 pub fn malloc_tuning_flag() {
     CommandLineTest::new()