refactor: change mria default shard transport from 'gen_rpc' to 'distr'

Erlang distribution seems to outperform gen_rpc (unless gen_rpc clients
are scaled up, but this is not easy to achive for shard transport as it
may reorder events).

apps/emqx_conf/src/emqx_conf_schema.erl

@@ -596,7 +596,7 @@ fields("node") ->
                 #{
                     mapping => "mria.shard_transport",
                     importance => ?IMPORTANCE_HIDDEN,
-                    default => gen_rpc,
+                    default => distr,
                     desc => ?DESC(db_default_shard_transport)
                 }
             )},