Blender Git Loki

Make 'check update' async, and add possibilities for engines to make async callbacks 'immediate'.

So now, jobs callback (async ones) of asset may return a specific job id value in case they actually
complete (or fail) on the first run.

This allows engines that do not need slow async stuff to perform an action to use simpler code
(e.g. imagine an engine able to list its assets from a cached DB, in most cases its list_dir
callback can execute instantaneously (from a user PoV), no need for a listing job then).