Added Database layer between saved vars and config/class config. Implemented automatic DB upgrade system.

To allow for database upgrades in future versions of EH, added a Database layer which provides access
to the saved vars for both config and class configs. A versioning schema/upgrade system is included
which allows for automatic upgrades of the config saved var structure.

Not yet implemented is class config versioning, though the structure is in place.

Other changes:
	Implemented Dijkstra's Pathfinding algorithm for the purposes of finding the
	optimal sequence of upgrade functions for automatic config db upgrades.

	Changed EventManager's startup tasks system to hide the implementation details
	and instead tasks should return true if the task completes successfully.
	If the task doesn't, then no return or a false return stops execution and prevents
	the addon's load with a customizable error message. (2nd return arg of task
	function is appended to error message.)