Implement WAIT FOR command

 Implement WAIT FOR command
 
 WAIT FOR is to be used on standby and specifies waiting for
 the specific WAL location to be replayed.  This option is useful when
 the user makes some data changes on primary and needs a guarantee to see
 these changes are on standby.
 
 WAIT FOR needs to wait without any snapshot held.  Otherwise, the snapshot
 could prevent the replay of WAL records, implying a kind of self-deadlock.
 This is why separate utility command seems appears to be the most robust
 way to implement this functionality.  It's not possible to implement this as
 a function.  Previous experience shows that stored procedures also have
 limitation in this aspect.
 
 Discussion: https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO+BBjcirozJ6nYbOW8Q@mail.gmail.com
 Discussion: https://www.postgresql.org/message-id/flat/CABPTF7UNft368x-RgOXkfj475OwEbp%2BVVO-wEXz7StgjD_%3D6sw%40mail.gmail.com
 Author: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
 Author: Alexander Korotkov <aekorotkov@gmail.com>
 Author: Xuneng Zhou <xunengzhou@gmail.com>
 Reviewed-by: Michael Paquier <michael@paquier.xyz>
 Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
 Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
 Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
 Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
 Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
 Reviewed-by: Euler Taveira <euler@eulerto.com>
 Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
 Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
 Reviewed-by: jian he <jian.universality@gmail.com>
 Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
 Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>