albatross_daemon keeps all unikernels in memory

[hannesm]

since a Unikernel.config contains a image : Cstruct.t - this is kept in memory during operation. There used to be (in ancient times) code to replace this with the empty cstruct once no longer needed, but since Unikernel_get allows to download the image, this is no longer the case.

Now, the albatross_daemon process running a lot of unikernels unnecessarily keeps the unikernel images in memory. We could (a) compress or (b) re-apply the old methodology and have the get command retrieve it from disk before sending it to the client. This would be very welcome.