config: setter/getter for Notifier and Logger into its own struct. (#3721)

This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.

Locks are now held properly not globally by configMutex, but
instead as private variables.

Final fix for #3700

cmd/notify-postgresql.go

@@ -174,7 +174,7 @@ func dialPostgreSQL(pgN postgreSQLNotify) (pgConn, error) {
 }
 
 func newPostgreSQLNotify(accountID string) (*logrus.Logger, error) {
-	pgNotify := serverConfig.GetPostgreSQLNotifyByID(accountID)
+	pgNotify := serverConfig.Notify.GetPostgreSQLByID(accountID)
 
 	// Dial postgres
 	pgC, err := dialPostgreSQL(pgNotify)