♻️ Short-circuit logout state transition

Not doing this with the other states, because the intention is to
eventually store small bits of information about each state on the
state objects, e.g: which mailbox was selected, what caused the logout.

For `logout`, the first attempt wins and shouldn't be overwritten.
For `selected`, the last attempt _should_ win.  (That said, commands
that affect connection state should be serialized.)

Author: nevans

lib/net/imap.rb

@@ -3797,7 +3797,9 @@ def state_unselected!
     end
 
     def state_logout!
+      return true if connection_state in [:logout, *]
       synchronize do
+        return true if connection_state in [:logout, *]
         @connection_state = ConnectionState::Logout.new
       end
     end