Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/notify-postgresql.go

@@ -242,7 +242,7 @@ func dialPostgreSQL(pgN postgreSQLNotify) (pc pgConn, e error) {
 }
 
 func newPostgreSQLNotify(accountID string) (*logrus.Logger, error) {
-	pgNotify := serverConfig.Notify.GetPostgreSQLByID(accountID)
+	pgNotify := globalServerConfig.Notify.GetPostgreSQLByID(accountID)
 
 	// Dial postgres
 	pgC, err := dialPostgreSQL(pgNotify)