sent_messages: Remove from cache when message has been fully received.

There is no reason to keep these messages in a local memory store once
we have completed sending them and seen their events.

web/src/sent_messages.js

@@ -96,6 +96,7 @@ export class MessageState {
         setTag("rendered_changed", this.rendered_changed);
         setTag("locally_echoed", this.locally_echoed);
         this.txn.finish();
+        messages.delete(this.local_id);
     }
 }