Create new git repo for ServiceComb Saga-Core module

As we Pack could be used for different coordinate protocols, it doesn't make sense that we host this module inside of Saga.
Please check this mail thread https://lists.apache.org/thread.html/1a846655f1320dabb9eec8dd879587d9e221d9bd593ba77bf709c2cd@%3Cdev.servicecomb.apache.org%3E for more information.