In recent sprint, we discussed that treemanifest should be moved out of
experimental. The feature has been used inside Google, Facebook in some forms
and was used inside Yandex too for narrow clones.
There is an issue with diffing of large treemanifests and to fix that some
Python code needs to be rewritted in C/Rust. This I think is not a blocker for
taking treemanifest out of experimental.
This patch renames config experimental.treemanifest to storage.treemanifest.