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.
This adds support for caching already rewritten trees, handling of load
errors and disabling the check that the serialization doesn't lead to
data loss.
The more generic RewriteNode callback replaces the SelectByName and
PrintExclude functions. The main part of this change is a preparation to
allow using the TreeRewriter for the `repair snapshots` command.
In principle, the JSON format of Tree objects is extensible without
requiring a format change. In order to not loose information just play
it safe and reject rewriting trees for which we could loose data.
