Check full backwards compatibility with libotr

[yakiradixon]

Why

The library should be backwards compatible with libotr.

References

See https://github.com/otrv4/otrv4/issues/48

Tasks

• [ ] Check what is missing from libotr to be "fully" compatible.
• [ ] Check that all callbacks are working.
• [ ] Check that everything from the bottom list is covered.

Open questions

None.

[claucece]

To check:

• [ ] Meta instance tags
• [ ] Fragmentation
• [ ] Context
• [ ] Resend messages by time: lastsent,lastrec
• [ ] OTR offer
• [x] 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.