pg_createsubscriber: Fix an unpredictable recovery wait time.

pg_createsubscriber: Fix an unpredictable recovery wait time.

The problem is that the tool is using the LSN returned by
pg_create_logical_replication_slot() as recovery_target_lsn. This LSN is
ahead of the current WAL position and the recovery waits until the
publisher writes a WAL record to reach the target and ends the recovery.
On idle systems, this wait time is unpredictable and could lead to failure
in promoting the subscriber. To avoid that, insert a harmless WAL record.

Reported-by: Alexander Lakhin and Tom Lane
Diagnosed-by: Hayato Kuroda
Author: Euler Taveira
Reviewed-by: Hayato Kuroda, Amit Kapila
Backpatch-through: 17
Discussion: http://postgr.es/m/2377319.1719766794%40sss.pgh.pa.us
Discussion: http://postgr.es/m/CA+TgmoYcY+Wb67NAwaHT7MvxCSeV86oSc+va9hHKaasE42ukyw@mail.gmail.com