The primary hash (aka the has used as primary key for the objstorage backend) used to be hardcoded as sha1, this makes it configurable among supported hashes (sha1, sha1_git, sha256 and blake2s256).
For the remote api to work, a new /config endpoint has been added
allowing a RPC client to ask the server for a config (generic dict), in
which the "primary_hash" key is used to configure the client with the same
primary hash as the server. If not such endpoint is available, defaults
to "sha1".