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.

beacon_node/src/config.rs

@@ -591,6 +591,10 @@ pub fn get_config<E: EthSpec>(
 
         slasher_config.broadcast = cli_args.is_present("slasher-broadcast");
 
+        if let Some(backend) = clap_utils::parse_optional(cli_args, "slasher-backend")? {
+            slasher_config.backend = backend;
+        }
+
         client_config.slasher = Some(slasher_config);
     }