Although we supported other hashes than SHA1 (which is now moderately unsafe)
we had to switch the metadata provider and daemon on some kind of flag day to
using SHA256. Since that's somewhat impractical, just allow multiple checksums
to be set on objects and just try to match whatever is given in preference
order.
This also means we can easily transition to other hash types in the future.
The removed API was never present in a tarball release, so not an API break.
