WIP: Fix M encoding + update protocol

Hello Kirill,

these patches aim to update the NEO/go protocol to support https://lab.nexedi.com/nexedi/neoppod/-/commit/9d0bf97a1327182ac29e95d65fd9e18742c43d1f.

There are two types of patches:

1. Fixing the msgpack encoding/decoding (continuing https://lab.nexedi.com/kirr/neo/-/commit/e8143557a22863430ba1a33b2516ee70a2441359)
2. Updating the protocol

The patches of 2. are related to the fact that https://lab.nexedi.com/nexedi/neoppod/-/commit/9d0bf97a1327182ac29e95d65fd9e18742c43d1f didn't only change the serialization, but also the protocol itself (as it's also noted in https://lab.nexedi.com/kirr/neo/-/commit/e8143557a22863430ba1a33b2516ee70a2441359). See:

1. https://lab.nexedi.com/levin.zimmermann/neoppod/-/commit/94a437643bcb2357572d6cebf43f188c58077425
2. https://lab.nexedi.com/levin.zimmermann/neoppod/-/commit/27a78346af19d44d65bdae3b804d7f584a941f8a

With this MR, tests against post-msgpack NEO/py pass on my machine (tests with the NEO/go server pass too).

From here, we could continue to update the NEO/go protocol to support the latest NEO/py protocol.

Best, Levin

/cc @kirr @klaus