Feature Request: Allow custom session handlers

Hi,

I've gained significantly speed by changing Hubzilla's session handler from the built-in database to a file based one (I've written a article discussing this on hubzilla: https://hub.netzgemeinde.eu/articles/buckaroo/ef2110301dc5c6e5)

I'd love to submit my patch - it works fine, if you don't set anything you'll still have the database as a session backend, you have to actively set it the another handler. Anyway, this patch gives admins the freedom of choosing which session handler to use - the built in or another, better suited one.Session.patch

[Edit]I've expenaded the patch, allowing the user to set gc_probabilty (which originally is set to 50) and gc_divisor , default now (if you use a custom session handler) is 1 and 100:

App::$config["system"]['session_gc_probability'] = 1; App::$config["system"]['session_gc_divisor'] = 100; Session2.patch