Better status/presence handling (T2109)

Summary:
Use client context to distinguish user presence "session".

TODO: The context string should probably be created on logon, so

using the same client in private browsing window will be
handled as a separate client

TODO: We still do not store the last used status, so when user connects

he becomes online.

Reviewers: Kolab Chat Developers

Differential Revision: https://git.kolab.org/D363