[BUGFIX] Cache results of explodeSoftRefParserList()

BackendUtility::explodeSoftRefParserList() is called with a very
limited set of input parameters, but processing is rather expensive.

Introduce a runtime cache to avoid processing the softrefs over
and over.
This especially speeds up calls from ReferenceIndex::getRelations(),
which is called many times.

Resolves: #63675
Releases: master, 6.2
Change-Id: I34a4392533828667c1b63b1ecb4ca12580e05e06
Reviewed-on: http://review.typo3.org/35190


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>