There was some duplicate code to run SHOW transaction_read_only to
determine whether the server is read-write or read-only.  Reduce it by
adding another state to the state machine.

Author: Hari Babu Kommi
Reviewed-by: Takayuki Tsunakawa, Álvaro Herrera
Discussion: https://postgr.es/m/CAJrrPGe_qgdbbN+yBgEVpd+YLHXXjTruzk6RmTMhqrFig+32ag@mail.gmail.com