Mark pg_start_backup and pg_stop_backup as parallel-restricted.

Mark pg_start_backup and pg_stop_backup as parallel-restricted.

They depend on backend-private state that will not be synchronized by
the parallel machinery, so they should not be marked parallel-safe.
This issue also exists in 9.6, but we obviously can't do anything
about 9.6 clusters that already exist.  Possibly this could be
back-patched so that future 9.6 clusters would come out OK, or
possibly we should back-patch some other fix, but that would need more
discussion.

David Steele, reviewed by Michael Paquier

Discussion: http://postgr.es/m/CA+TgmoYCWfO2UM-t=HUMFJyxJywLDiLL0nAJpx88LKtvBvNECw@mail.gmail.com