Make the session handlers all compatible with SessionHandlerInterface

[paulbalandan]

Description
Related to #4883

Reference for the updated signatures: https://www.php.net/manual/en/class.sessionhandlerinterface.php
I've refactored and cleaned the code a bit as I see fit.

My only concern is for the gc method. Our current implementation is using the pre-7.1 behavior where it returns true on success. Starting PHP 7.1, on success it should return the number of garbage collected sessions. My initial solution is to turn true to 1 for handlers where the number can't easily be determined while returning the true number on others (like the file handler).

Checklist:

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