[!!!][FEATURE] Introduce Session Framework

A new session framework is introduced.
The goal is to provide interoperability between different
session storages (called "backends"), like database, Redis, etc.

An integrator may enforce a specific session backend by configuring
SYS/session in LocalConfiguration.php. It is also possible to use
custom session backends by implementing
the interface "SessionBackendInterface".

Resolves: #70316
Releases: master
Change-Id: I90a4f84344e75f13b2f46245162e749ed3505ec3
Reviewed-on: https://review.typo3.org/51549


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>