[TASK] Add cache to getPagesTSconfig()

Add a cache also to BackendUtility::getPagesTSconfig() to save parsing
etc. for multiple lookups in various places inside TYPO3.

The core only uses the first parameter, therefore the cache is only
based on the page ID and ignored if other parameters are given.

Change-Id: I63d49c398ed16daa5c2d8ae707d67f934a24a312
Resolves: #53462
Releases: 6.2
Reviewed-on: https://review.typo3.org/25223
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind