[BUGFIX] Support concurrent requests without 503 responses

Instead of offloading the work to wait for the final page content,
concurrent requests now wait for the real page content to be
rendered (and deliver the content from cache once ready) instead
of sending a 503 response code and the famous "Page is being
generated" message.

The logic to wait for the rendered page (lock and wait) is already
there thanks to the page locking, but was deliberately circumvented
by the temporary page cache content before to get rid of waiting
requests in high-load situations.

This approach to simply skip the temporary cache and wait for the
renderer to finish has been tested using the skip_page_is_being_generated
extension in wild for 3 years (for TYPO3 v6, v7, v8 and v9).

Note: In case the increased number of waiting requests has a negative
impact on sites with high server load, a additional proxy cache should be
considered in front of the server to make sure clients are served a valid
response without waiting until new content is ready.

The motivation for this change:
The 503 status code together with the "Page is being generated message"
does not only occur for slow or high traffic sites. It will be displayed
even for two concurrent requests, no matter how fast the page rendered
or how low the current traffic is.
The requests only need to (nearly) arrive at the same time. This can
easily be reproduced using two parallel curl requests:
for i in {1..2}; do curl -sv https://doma.in/foo |& grep '^< HTTP'& done
There would be one "503 Service unavailable" response when /foo has not
yet been rendered to the cache before.

An explanation for the releaseLock('pagesection') addition in
TSFE::getFromCache(): This has been added as the pagesection lock – which
is acquired in TemplateService::start() – was implicitly released in
setPageCacheContent() before. Now this would block concurrent rendering
for pages with $_GET-aware plugins, therefore we release the pagesection
lock early, after the pagesection cache has been generated.

Releases: master, 9.5, 8.7
Resolves: #87980
Change-Id: I034f410335b3035c5863b26e3e689ca29b5f3f80
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60307


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>