During write, either the ModificationDiff or Writers need to be schema aware. For instance, they need to know whether a container is a presence container or not to pass propagate empty container down the pipeline.

This commit makes ModificationDiff schema aware: https://gerrit.fd.io/r/#/c/3491/ ... this works, but it is slow and would require caching to make it bearable. We can improve by moving the schema awareness down the pipeline by making writers schema aware. The Registry would process schema context when building the registry and assign a schema node to each writer. This would improve performance since the schema tracking is not necessary and the schema context will only be traversed once. Also, this would make it possible to have schema awareness symmetric for readers and writers once we have to make Readers schema aware too.