optim: preallocate base-image placeholders

[rgaudin]

image-creator spends time on each task creating the final image from the base image ; either:

• downloading and extracting it
• extracting from cache
• copying from non-compressed cache

Copying those few GB of data takes a number of seconds for something we know in advance we'll need.

We could imagine the worker keeping a small pool of ready to move/rename copies of the base image and hand them over to the image-creator.

[kelson42]

@rgaudin Or put them in the cache?

[rgaudin]

re-read the ticket?