Clarify multiplexing profile operations is not allowed in Certz.Rotate

[robshakir]

* (M) certz/certz.proto
   - It is possible within the current certz API for a client to start
     an operation for ssl_profile_id = "foo" and then, using the same
     RPC, start an operation for ssl_profile_id = "bar". This
     introduces significant server-side complexity since it must track
     the state machine on a per-profile basis, with little benefit to
     the client. Clarify that this is not allowed and specify the
     error code to indicate as such.