Session: Rework logic between init(...), getInstance(...) and __construct(...).

This should make that code much easier to understand, and it fixes a few
hard-to-hit bugs, such as the authority not being set if the session doesn't
exist before being created.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@565 44740490-163a-0410-bde0-09ae8108e29a