Revert to old SESSION global behavior

This makes it so the global factory storage for SESSION isn't reset every time a handler is passed. To update the global storage, change the script id, or call SESSION.clearAll() and register your script and factories again.

Currently, it constructs the global factory after calling serverStartUp (this was the exact old behavior), but it's possible to do this right away (after the handler is passed).

Fixes #881.