Fixing problem with multiple windows.
Garbage collection
Adding garbage collection for encrypted state.
Trying a different approach, where formstate is stored per window. This uses the previous versions ability to reuse a formstate that will become obsolete. It also adds a garbage collection limit, and changes the form response if the user does happen to backup past the limit, or the session expires, to something more reasonable than throwing an exception.
Merge pull request #194 from spekary/bugfix-SessionFormStateHandler2

A new attempt at bugfix for session formstate handler