Real time data integrity check during broadcast time

This is a sub task of ZOOKEEPER-3114, which is going to calculate the hash value of data tree after each txn, leader will proposal txn with digest, and when learner applied the txn to data tree, it will check if it has the same hash value or not.

Currently, the default behavior is logging and reporting via metric, people can also implement auto-recover based on the event received from DigestWatcher.