[FEATURE] Introduce xcache cache backend

xcache is another PHP opcode cache like APC.
Similar to APC, it has a key-value for user data.
This storage is now available in the TYPO3 caching framework
by providing an according cache backend.

Note: XCache does not work in CLI mode!

Resolves: #46142
Releases: 6.1
Change-Id: I4e9e856a8397978eb6f6d480b79996ff49ff8a7b
Reviewed-on: https://review.typo3.org/18823
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn