Always serialize trees via TreeJSONBuilder. Add a wrapper called
TreeWriter which combines serialization and saving the tree blob in the
repository. In the future, TreeJSONBuilder will have to upload tree
chunks while the tree is still serialized. This will a wrapper like
TreeWriter, so add it right now already.
The archiver.treeSaver still directly uses the TreeJSONBuilder as it
requires special handling.
data.TestCreateSnapshot which is used in particular by TestFindUsedBlobs
and TestFindUsedBlobs could generate trees with duplicate file names.
This is invalid and going forward will result in an error.
The new method combines both step into a single wrapper function. Thus
it ensures that both are always called in pairs. As an additional
benefit this slightly reduces the boilerplate to upload blobs.
