postgres pgx driver not supported with placeholder function

Extract of fix config which I am working at

PersistMessages=Y
SQLStoreDriver=pgx
SQLStoreDataSourceName=postgres://pgxuser:pgxpass@localhost:5432/testdb

Using the above session settings is not able to initiate the session, having the postgres module as https://github.com/jackc/pgx which registers the driver as pgx https://github.com/lib/pq - This module registers the driver as postgres. pq repo is not active and they recommend using pgx driver.

postgresPlaceholder can be used only if the driver is postgres and does not support pgx. Code

	if store.sqlDriver == "postgres" {
		store.placeholder = postgresPlaceholder
	}