An authenticated connection should not be allowed to reauthenticate

A client may currently do something like:

SASL AUTH user1

   run commands as user 1

SASL AUTH user2

   run commands as user 2

None of our clients currently do this, and it makes the lifecycle of a connection unnecessary complex as the connection may also transition from "authenticated" to "unauthenticated" (as the SASL authentication may involve multiple steps and also fail).