allow to use application name for upgrade

Instead of using replica IP address, allow to use replica name in case IP address is not reliable (like when using a service mesh).
This trigger is controller by using an environment variable and default behavior is the previous one

postgres-appliance/major_upgrade/inplace_upgrade.py

@@ -170,14 +170,19 @@ def ensure_replicas_state(self, cluster):
         to all of them and puts into the `self.replica_connections` dict for a future usage.
         """
         self.replica_connections = {}
+        selector = "client_addr"
+        if os.getenv('USE_APPLICATION_NAME_IN_UPGRADE'):
+            selector = "application_name"
         streaming = {a: l for a, l in self.postgresql.query(
-            ("SELECT client_addr, pg_catalog.pg_{0}_{1}_diff(pg_catalog.pg_current_{0}_{1}(),"
+            ("SELECT {2}, pg_catalog.pg_{0}_{1}_diff(pg_catalog.pg_current_{0}_{1}(),"
              " COALESCE(replay_{1}, '0/0'))::bigint FROM pg_catalog.pg_stat_replication")
-            .format(self.postgresql.wal_name, self.postgresql.lsn_name))}
+            .format(self.postgresql.wal_name, self.postgresql.lsn_name, selector))}
 
         def ensure_replica_state(member):
             ip = member.conn_kwargs().get('host')
             lag = streaming.get(ip)
+            if os.getenv('USE_APPLICATION_NAME_IN_UPGRADE'):
+                lag = streaming.get(member.name)
             if lag is None:
                 return logger.error('Member %s is not streaming from the primary', member.name)
             if lag > 16*1024*1024: