[Azoth] Removed unneeded slot.
[Azoth] ID-based server history for #1536.

Offsets aren't necessarily numeric-based. Make them abstract QByteArray
IDs.
[Azoth] Support filtering history contacts view for #1536.
[Azoth Xoox] MAM archive query iq for #1536.
[Azoth Xoox] Xep0313Manager::RequestHistory() for #1536.
Merge branch 'master' of github.com:0xd34df00d/leechcraft