Handle temporarily unavailable annex content

[mpsonntag]

If (annexed) content is not available yet, wait and retry. We would have to set a max timeout for this. The issue is that when a user does a 'gin upload' a push happens immediately and the hook is triggered, but annexed content is only transferred after the push and could take a while (hours?). The validation service should try to download content after the transfer is complete, or should keep retrying until it's available, with a timeout. We could also make it more efficient by only downloading the content in the directories which are specified in the validator config (if it exists).