Adds automatically background refresh of all external links if they are belongs to the current blob (S3) storage. The feature is disabled by default in order to keep backward compatibility.
The background go-routine spawns once during startup and periodically signs and updates external links if that links belongs to current S3 storage.
The original idea was to sign external links on-demand, however, with current architecture it will require duplicated code in plenty of places. If do it, the changes will be quite invasive and in the end pointless: I believe, the architecture will be eventually updated to give more scalable way for pluggable storage. For example - Upload/Download interface without hard dependency on external link. There are stubs already, but I don't feel confident enough to change significant part of the application architecture.
