Implement Health API

[rahst12]

The Health API offers insight into cluster operations and health. While performing massive ingest, if a predicate is moving between groups, it dramatically slows down ingest when writing the predicate that is being moved. We'd like to be able to proactively disable, slowdown, or otherwise take action with the continuous ingest stream when operations like this occur.

https://docs.dgraph.io/admin/admin-tasks/check-cluster-health

Thanks