A correct support of OMEMO: 12 byte for initilization vector (IV)

[Neustradamus]

The XEP-0384 0.3.0 does not specify the "IV" value and the 0.3.1 version has been missing before the 0.4.0 version.
All OMEMO 0.3.0 clients must send with 12-byte instead 16-byte IV.

This issues is solved in gkdr/libomemo#27.

More informations:

• https://www.google.com/search?q=12+byte+iv
• https://monal.im/blog/omemo-and-french-laws/
• https://github.com/ChatSecure/ChatSecure-iOS#127
• https://monal.im/blog/omemo-is-broken-in-general-across-the-ecosystem/

Already good:

• Psi/Psi+: https://github.com/psi-im/plugins/search?q=12+iv
• Profanity: https://github.com/profanity-im/profanity/search?q=12+iv
• Conversations: https://github.com/iNPUTmice/Conversations/search?q=12+iv
• Gajim: https://github.com/gajim/gajim-plugins/search?q=12+iv
• Dino: https://github.com/dino/dino/search?q=12+iv
• ChatSecure: https://github.com/ChatSecure/ChatSecure-iOS/search?q=12+iv
• Monal: https://github.com/anurodhp/Monal/search?q=12+iv
• Tigase Siskin IM: https://github.com/tigase/siskin-im/search?q=12+iv
• Tigase Swift Omemo: https://github.com/tigase/tigase-swift-omemo/search?q=12+iv
• aTalk (via Smack): https://github.com/cmeng-git/atalk-android/search?q=12+iv
• Smack (lib): https://github.com/igniterealtime/Smack/search?q=12+iv
• jsxc: https://github.com/jsxc/jsxc/search?q=12+iv
• Converse.js: https://github.com/conversejs/converse.js/search?q=12+iv
• Python Omemo: https://github.com/Syndace/python-omemo/search?q=12+iv

Linked to:

• 12 byte IV for outgoing messages libomemo#24
• add defined IV size for GCM. clarified text xsf/xeps#894

[Neustradamus]

@gkdr: I am sorry but can you release the lurch 0.7?

Some OS has not separated libomemo/axc/lurch and have not currently the fix.

Thanks in advance.

[gkdr]

done with 73d7c00