Milestone: provide alternative APIs without CheckedFuture

Guava has deprecated checked future in its current (to-be-22.0) codebase: https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/util/concurrent/CheckedFuture.html.

This puts us on the spot when the time comes to upgrade ("This class is scheduled for removal from Guava in February 2018"), both yang-model-api and yang-data-api are affected.