storagemanager: Add keyval+ (combined keyval + map) store type This combines the two most common store types, which modules often end up opening with both interfaces separately anyway. As well as combining them, I've taken the opportunity to improve some of the method names to make them clearer.

local record util_timer
	record task end
	type timer_callback = function (number) : number
	add_task : function ( number, timer_callback, any ) : task
	stop : function ( task )
	reschedule : function ( task, number ) : task
end
return util_timer