cache local instance (#572)

* cache local instance

fix #564

* don't use local instance cache for plm

* use instance cache for plm, but initialize it

* cargo fmt

plume-cli/src/users.rs

@@ -129,7 +129,7 @@ fn reset_password<'a>(args: &ArgMatches<'a>, conn: &Connection) {
     let user = User::find_by_name(
         conn,
         &username,
-        Instance::get_local(conn)
+        Instance::get_local()
             .expect("Failed to get local instance")
             .id,
     )