[TASK] Clean up DataHandler

Some cleanup of the class with obvious things:

* Add ConnectionPool, CacheManager and runtime cache as
  property, created in __construct(), can be DI later.
* Remove unused getResourceFactory(), remove
  getCacheManager() and getRuntimeCache() since they
  are properties now.
* Add type hints to most properties: Those that can be
  set without headaches since DH would crash if they'd
  not be of that type, or other obvious reasons.
* Turn a protected magic-string property into a constant
  and add another related constant to avoid a magic-string
  in code.
* Add a series of method return types: Obvious ones where
  the method clearly can't return something else.
* Minor comment spelling improvements.

Change-Id: I117548d2ed44236208063774680079578e8392b5
Resolves: #103135
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83019


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>