Timeline entity uniqueness

In YARN-3051, we have some discussion about how to uniquely identify an entity. Sangjin and some other folks propose to only uniquely identify an entity by <type, id> in the scope of a single app. This is different from entity uniqueness in ATSv1, where <type, id> can globally identify an entity. This is going to affect the way of fetching a single entity, and raise the compatibility issue. Let's continue our discussion here to unblock YARN-3051.