[BUGFIX] Rework AJAX request queueing

This patch reworks the queueing system for AJAX requests sent by the
extension scanner. This implementation now really only sends the allowed
amount of concurrent requests and queues the remaining requests.

When the extension scanner gets closed, the queue gets flushed and the
remaining running requests get aborted. Since aborting a request triggers
a DOMException, the rather useless "Oops" notification gets removed.

The extension scanner now also queues requests where it actually makes
sense.

Resolves: #90279
Releases: master, 9.5
Change-Id: Ia5a64a2074fd99b8447063e2135126b7cf46e393
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63107


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>