This is safer: in the earlier API, an old webapi server would silently ignore
the initial children, and clients trying to set them would have to fetch the
newly-created directory to discover the incompatibility. In the new API,
clients using t=mkdir-with-children against an old webapi server will get a
clear error.
interfaces.py: define INodeMaker, document argument values, change
create_new_mutable_directory() to take dict-of-nodes. Change
dirnode.set_nodes() and dirnode.create_subdirectory() too.
nodemaker.py: use INodeMaker, update create_new_mutable_directory()
client.py: have create_dirnode() delegate initial_children= to nodemaker
dirnode.py (Adder): take dict-of-nodes instead of list-of-nodes, which
updates set_nodes() and create_subdirectory()
web/common.py (convert_initial_children_json): create dict-of-nodes
web/directory.py: same
web/unlinked.py: same
test_dirnode.py: update tests to match
Now upload or encode methods take a required argument named "convergence" which can be either None, indicating no convergent encryption at all, or a string, which is the "added secret" to be mixed in to the content hash key. If you want traditional convergent encryption behavior, set the added secret to be the empty string.
This patch also renames "content hash key" to "convergent encryption" in a argument names and variable names. (A different and larger renaming is needed in order to clarify that Tahoe supports immutable files which are not encrypted content-hash-key a.k.a. convergent encryption.)
This patch also changes a few unit tests to use non-convergent encryption, because it doesn't matter for what they are testing and non-convergent encryption is slightly faster.
