Check how instance tags are managed

[claucece]

Why

As we are currently implementing the revision number 2 of the OTRv4 specification, we need to include a consistent way of using Instance Tags.

Reference

Please, refer to the "Instance Tags" section of the OTRv4 spec.

Tasks

• [x] Check how instance tags are managed.
• [x] Check how instance tags are been get from the client.
• [x] Correctly use the callbacks for instance tags.
• [x] Correctly test and use them over tests.
• [x] Check how they are used over libotr3 and if we can use them in a similar way.
• [x] Support multiple conversations with the same recipient if they use multiple instance tags. See TODO on line 63 of src/client.c UPDATE: this is for the plugin

Open questions

None.

[claucece]

On this, check

The smallest valid instance tag is `0x00000100`. It is appropriate to set the
destination instance tag to `0` when an actual destination instance tag is not
known at the time the message is prepared. If a client receives a message with
the sender instance tag set to less than `0x00000100`, it should discard the
message. Similarly, if a client receives a message with the recipient instance
tag set to greater than `0` but less than `0x00000100`, it should discard the
message.

[claucece]

Anything related with the client and otrv3 will be checked in the next project ;)