Clients should identify services by nickname, not hsid

• Add a client-side ClientSideFooHsNickname type. Pick a name for it (it should be distinguishable from the service-side HsNickname), define its charset and decide what limit to impose on its length
• Add a ClientSideFooHsNickname to the client key specifiers, and make clients error if there is more than one nickname for any given HsId