CDN Warmer always waits for the first Promise in the batch

Problem/Motivation

The CDN warmer code has the following statement for limiting concurrent requests:

      $item_keys = array_keys($items);
      if ($key % $max_concurrent_requests == 0 || $key == end($item_keys)) {
        \GuzzleHttp\Promise\all($promises)->wait();
        $promises = [];
      }

However, if the $key is 0 (i.e. first URL in the list), the $key % $max_concurrent_requests == 0 expression is always TRUE.

As a result, it's always waiting for the first URL in the list to be fetched before proceeding with the rest of the batch (which could be then fetched concurrently as expected).

Proposed resolution

Patch attached.

Remaining tasks

Review and merge.

User interface changes

None.

API changes

None.

Data model changes

None.