Don't retreat slot's confirmed_flush LSN.

Don't retreat slot's confirmed_flush LSN.

Prevent moving the confirmed_flush backwards, as this could lead to data
duplication issues caused by replicating already replicated changes.

This can happen when a client acknowledges an LSN it doesn't have to do
anything for, and thus didn't store persistently. After a restart, the
client can send the prior LSN that it stored persistently as an
acknowledgement, but we need to ignore such an LSN to avoid retreating
confirm_flush LSN.

Diagnosed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Author: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Tested-by: Nisha Moond <nisha.moond412@gmail.com>
Backpatch-through: 13
Discussion: http://postgr.es/m/CAJpy0uDZ29P=BYB1JDWMCh-6wXaNqMwG1u1mB4=10Ly0x7HhwQ@mail.gmail.com
Discussion: http://postgr.es/m/OS0PR01MB57164AB5716AF2E477D53F6F9489A@OS0PR01MB5716.jpnprd01.prod.outlook.com