message_fetch: Add small backfill delay.

This should help reduce the risk of hitting rate limits when users
have a very large number of messages to fetch via this mechanism.

Inline the `messages` variable that was only used in one place while
we're touching this.

web/tests/message_fetch.test.js

@@ -222,6 +222,12 @@ function forward_fill_step() {
     let fetch;
 
     self.prep = () => {
+        /* Don't wait for the timeout before recursively calling `load_messages`. */
+        const expected_delay = 150;
+        set_global("setTimeout", (f, delay) => {
+            assert.equal(delay, expected_delay);
+            f();
+        });
         fetch = config_fake_channel({
             expected_opts_data: initialize_data.forward_fill.req,
         });