Add modSessionHandlerInterface

[theboxer]

What does it do?

• Creates a modSessionHandlerInterface interface and uses it for modSessionHandler
• Moves flushSessions method from Security\Flush processor to the interface & handler
• In the Security\Flush processor - checks if flushSessions method exists in the handler class stored in session_handler_class system setting and executes it if so

Why is it needed?

Custom session handlers can't flush all sessions.

How to test

#15928

Related issue(s)/PR(s)

Resolves #15928

[JoshuaLuckers]

@sergant210 can you check if this PR solves your issue?

[theboxer]

btw. this is kind of duplicate of #15934 I didn't notice that PR sooner

[sergant210]

@JoshuaLuckers My PR solves my issue ;)

[opengeek]

My PR is solves my issue ;)

This was not the question.

[JoshuaLuckers]

@theboxer can you explain the main difference between this PR and #15934 ? I don't have enough time to look into the details to decide which one we should approve and merge.

[Ibochkarev]

Up

[rthrash]

This pull request has been mentioned on MODX Community. There might be relevant details there:

https://community.modx.com/t/error-flush-sessions-not-supported-when-trying-to-logout-all-users-in-modx-3/6361/2

[Mark-H]

Closing in favour of #16522 - please see my explanation there.