The TreeNodeIterator decodes nodes while iterating over a tree blob.
This should reduce peak memory usage as now only the serialized tree
blob and a single node have to alive at the same time. Using the
iterator has implications for the error handling however. Now it is
necessary that all loops that iterate through a tree check for errors
before using the node returned by the iterator.
The other change is that it is no longer possible to iterate over a tree
multiple times. Instead it must be loaded a second time. This only
affects the tree rewriting code.
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.
