Check full backwards compatibility with libotr about libotr-ng

[claucece]

To check:

• Meta instance tags
• Fragmentation
• Context
• Resend messages by time: lastsent,lastrec
• OTR offer
• Check OtrlSessionIdHalf for ssid
• Old mac keys on context_priv
• Other error codes from otrv3
• Logged in
• Disconnect all instances
• otrl_message_poll_get_default_interval
• privkey.h
• tlv.h
• version.h and m4
• possibly: #14
• priv, fingerprints from libotr3

[claucece]

Also check:

• fingerprint_seen_otr3 seems a bit shortsighted. maybe we should just give a "version" argument to fingerprint_seen instead?
• Implement or check most of the otrv4_client_callbacks_ functions

[olabini]

When it says backwards compatible, it doesn't actually mean backwards compatible - it means all the functionality should still work and be available, but not with the same API.

[olabini]

After discussion about this, most of this is probably done - although we need to make sure the otr3 callbacks work correctly.