search: Remove buggy double-call of set_message_booleans.

In a refactor last fall, we changed `set_message_booleans` to mutate
state (specifically, destroying msg.flags in favor of setting
properties like `msg.unread`).  This was fine for most code paths, but
the maybe_add_narrowed_messages code path called
`message_store.add_message_metadata` twice (once after talking to the
server to find out whether the messages go into the current narrow),
and so when we extracted set_message_booleans from that, the second
call didn't properly short-circuit.

We fix this by just removing the second call, and also add a comment
warning about the add_message_metadata call there as being dangerous.

Fixes #8184.

static/js/message_events.js

@@ -31,8 +31,14 @@ function maybe_add_narrowed_messages(messages, msg_list, messages_are_new) {
                 }
             });
 
-            _.each(new_messages, message_store.set_message_booleans);
+            // This second call to add_message_metadata in the
+            // insert_new_messages code path helps in very rare race
+            // conditions, where e.g. the current user's name was
+            // edited in between when they sent the message and when
+            // we hear back from the server and can echo the new
+            // message.  Arguably, it's counterproductive complexity.
             new_messages = _.map(new_messages, message_store.add_message_metadata);
+
             message_util.add_messages(
                 new_messages,
                 msg_list,