perf: coordinate unknown id lookups

This is a huge improvement over what it was before. Now all unknown id
lookups are debounced and happen through a central coordinator. This
ensures there is no duplication between timelines.

Fixes: https://github.com/damus-io/notedeck/issues/279
Signed-off-by: William Casarin <jb55@jb55.com>

src/lib.rs

@@ -35,6 +35,7 @@ mod time;
 mod timecache;
 mod timeline;
 pub mod ui;
+mod unknowns;
 mod user_account;
 
 #[cfg(test)]