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-models/src/comments.rs

@@ -79,7 +79,7 @@ impl Comment {
     pub fn count_local(conn: &Connection) -> Result<i64> {
         use schema::users;
         let local_authors = users::table
-            .filter(users::instance_id.eq(Instance::get_local(conn)?.id))
+            .filter(users::instance_id.eq(Instance::get_local()?.id))
             .select(users::id);
         comments::table
             .filter(comments::author_id.eq_any(local_authors))
@@ -107,7 +107,7 @@ impl Comment {
         let author = User::get(&c.conn, self.author_id)?;
         let (html, mentions, _hashtags) = utils::md_to_html(
             self.content.get().as_ref(),
-            Some(&Instance::get_local(&c.conn)?.public_domain),
+            Some(&Instance::get_local()?.public_domain),
             true,
             Some(Media::get_media_processor(&c.conn, vec![&author])),
         );