The downloaded source file should be re-used if the hash hasn't changed

When the Sip-Creator requests a sip.zip file for download, the hub needs to build the source XML anew form basex and carefully re-construct it the exact same way that it was sent in, because otherwise the hash is different when it gets back on the client.

This process can take a long time, especially for large sets. Instead of repeating the operation at every time, we should look into caching the result of this operation (source data serialization) and re-use it if the hash has not changed.