add save anon session feature

[juanetch]

What?

Added app configuration option to enable/disable saving anonymous user sessions.

Why?

Feature adds more control to a developer over behavior of session based cookies. Whether for performance (ex. bloating session storage with anonymous user sessions) or for regulatory reasons (ex. GDPR and allowing cookies), a developer can now choose to set a session based cookie when required (ex. after authenticating).

How?:

If disabled - no session is set when request arrives, instead, function access to SessionManager is provided through req->setSessionManager() and then req->setSession polymorphism will work. req-setSession() returns boolean if SessionManagerPtr is available since it might not be if config enable_session is false. We may also just call req-session() and if SessionPtr is not available it will try to setSession using the SessionManager.

Testing?

• Client integration: Remove Get Cookie test and move it to Test Session (this is the test that makes use of session).
• TimeFilter - if sessionId is not present - it is created through req->setSession().

Anything Else?

• Enable/Disable session could potentially be removed since with this feature since it allows developer to set session when required.
• Moved SessionManager.h to INCLUDES because HttpRequest.h requires it - not sure if this is correct.
• Providing access to SessionManager in HttpAppFrameworkImpl by passing a naked pointer (SessionManager is a unique_ptr) was done for performance considerations (using shared_ptr could eat up resources if many request share pointer), but could be negligible as I did not test performance - not an expert on this.
• Not sure if I misused virtual functions.