logspec: redirect needs scope

[superfell]

When the server returns a redirect reply, it's not currently defined whether that's supposed to affect the current request type, all request types, or some subset. This could get us into trouble. For example, consider a client that was issuing Appends() as well as a Read() for an early prefix of the log. If the servers redirected Appends to a leader but old reads to a follower, the client could be bounced back and forth.