[Session Handlers] Properties protected to be able to extend them

Hello,

I need to override one method from the mongodb session handler, so, I would like to extend it to keep the other methods, but that's not really possible without copy/past some methods because of the "private" methods & attributes.

So, I changed all session handlers private attributes & methods to protected, to be able to extend them.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no    #Not really a new feature
| BC breaks?    |no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 


Regards,
Peekmo