[BUGFIX] Make persistent connections available for Redis cache backend

The Redis cache backend allows configuration of all connection
setting, except setting persistent connections. This omission
is fixed by this patch. Being unable to use persistent connections
under high load can hurt performance in cloud setups. Persistent
connections has been part of phpredis since 2010 and thus is
broadly available, but is impossible to use with TYPO3.

Resolves: #79005
Releases: master, 7.6
Change-Id: Ie5476e894072bb9979691f28bc7a1e10e86ef454
Reviewed-on: https://review.typo3.org/50978


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Kasper Ligaard <kasperligaard+typo3.org@gmail.com>
Reviewed-by: David Greiner <hallo@davidgreiner.de>
Tested-by: David Greiner <hallo@davidgreiner.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>