Blender Git Loki

Change how Amber (and all other engines) are expected to work.

So far, we were accepting the idea of several repositories per asset engine (e.g. different
directories in case of Amber). However, if this works OK during listing/initial linking,
it makes things way more complicated later in asset management, since asset uuid is no more
enough to unically indentify an asset!

Now an asset UUID is assumed to be really unique inside an asset engine.

For Amber, we 'salt' assets' uuids with new repo uuids (both being only 2 ints long now),
and keep a config 'cache' of mapping repo uuid -> path to repo.