Fix handling of larger messages in ockam_node

[thomcc]

Currently, we don't handle messages above a certain size correctly in the rust https://github.com/ockam-network/ockam/blob/bbb2f85bbb561095497b701b50e8e005f301b518/implementations/rust/ockam/ockam_node/src/parser.rs#L17

This doesn't correctly implement the BARE variable length integer scheme described in https://www.ietf.org/archive/id/draft-devault-bare-01.txt:

Each octet of the encoded value has the most-significant bit set, except for the last octet. The remaining bits are the integer value in 7-bit groups, least-significant first.

(Note that this means the value we can't be higher than is 127, not 255 as mentioned in the linked source, because of how the encoding works — if it were 128 or larger, the top bit would be set, meaning we would only be supposed to use the bottom 7 from this octet, and check the next one for the rest.)

It's possible this is the cause of some of the errors that we're seeing in #1624 (it seems somewhat surprising to me that we wouldn't even have "in-order for the most part in practice", but I could be mistaken or even misunderstanding what's being described). Either way, we should fix it.

It's possible this can be done by just using serde_bare. Not sure. If not, the varint scheme isn't that complex.

[neil2468]

Sounds interesting. Can I be assigned this?

[thomcc]

All yours!