This commit adds new GUC (hidden) to control if ctid can be used
while updating replicated tables.   By default, such statement
will cause an error.

Submitted by Abbas Butt.
This is a tentative fix for pgxc_ctl to deal with the following
failure in the current Postgres-XC.

1) pg_basebackup does not work with coordinator.
2) ALTER NODE tries to connect to the pooler, which should not happen.

Both was caused by unnecessary call to the pooler.   This fix will be
provided in the separate patch.