Restrict identifier APIs to only using prefixes

[iFergal]

Identifier resources in KERIA can now be referenced by either alias (like before) or prefix, like so: hab = agent.hby.habs[name] if name in agent.hby.habs else agent.hby.habByName(name)

It would be cleaner and more intuitive to move to only supporting a prefix, especially since the alias can be renamed. As pointed out by @lenkan using PUT /identifier/name1 to update that resource to now be available at /identifier/name2 is unintuitive.