AliasManagerCacheDecorator won't work correctly with subrequests

The problem that Crell ran into in trying to address request scope dependency issues in #1874500: CMF-based Routing system is one that I mistakenly ignored when working on #1269742: Make path lookup code into a pluggable class - which means the code in there is faulty :-(

  public function writeCache() {
    $path_lookups = $this->getPathLookups();
    // Check if the system paths for this page were loaded from cache in this
    // request to avoid writing to cache on every request.
    if ($this->cacheNeedsWriting && !empty($path_lookups) && !empty($this->cacheKeys)) {
      // Use the system path of the current request for the cache ID (cid).
      $cid = end($this->cacheKeys);
      // Set the path cache to expire in 24 hours.
      $expire = REQUEST_TIME + (60 * 60 * 24);
      $this->cache->set($cid, $path_lookups, $expire);
    }
    // We are at the end of the request, so pop off the last request path.
    array_pop($this->cacheKeys);
  }

That last comment is completely delusional - this gets called when the kernel terminates, *not* at the end of a request. There is currently no way to make sure this gets called at the end of each (sub)request. But apparently fabpot is working on a solution: https://github.com/symfony/symfony/issues/5300

In the meantime we should try and come up with something that is at least less utterly false.