Cluster transaction pipelining requires yangtools support

I have finished the cluster transaction pipelining function and have done performance test in boron, the tx rate increased greatly. The function involved the following projects.

1. sal-akka-raft
2. sal-distribute-datastore
3. yangtools(yang-data-api yang-data-impl)

The whole function based on the yangtools's new api, so need to submit the yangtools code at first