Split backend identifiers from the class name

Prior to this, the storage class name was stored in mount.json under the
"class" parameter, and the auth mechanism class name under the
"authMechanism" parameter. This decouples the class name from the
identifier used to retrieve the backend or auth mechanism.

Now, backends/auth mechanisms have a unique identifier, which is saved in
the "backend" or "authMechanism" parameter in mount.json respectively.
An identifier is considered unique for the object it references, but the
underlying class may change (e.g. files_external gets pulled into core
and namespaces are modified).