handle OTRv3 Instance Tags

[afflux]

instance tags are 2x 4-byte identifier, located in every messageright after the message type field.

• provide API for handling instance tags
• conditionally fallback to OTRv2-style messages if remote side doesn't support instance tag

[varac]

Ugh, no progress since 5 years ?

[koolfy]

Actually the priority should be changing the crypto lib to cryptodome (there was still some work needed for that, regarding CTR counter management, that I didn't do yet :( ), and probably adding more unit tests? Help would be appreciated, actually…