Add branch for future WC2 compatibility mode

Between NEO/go and NEO/py there are various incompatibilities.
Similarity to the 'wc2' branch [1], this branch aims to transparently
communicate those incompatibilities.

Unlike the 'wc2' branch, we apply those patches here on an up-to-date
recent NEO/py. The background is that current NEO/go version still uses
an old pre-msgpack protocol and didn't fully implement the new msgpack
protocol yet. So the 'wc2' branch still builds upon an old NEO/py
version (1.12, from the oldproto branch). The diff between the
wc2-future branch and master on the other hand aims to be as minimal
as possible and to only contain the compatibility patches.

[1] https://lab.nexedi.com/nexedi/neoppod/tree/wc2 and nexedi/neoppod@739096b7