Updates smack 4.2.1 with some fixes.
Updates smack 4.2.1 with some fixes.

Fixes memory leaks inside MultiUserChat. The source code and fixes are
in https://github.com/jitsi/Smack/tree/4.2.1-jitsi. The maven artifacts
are using that repo.
These are the corresponding PRs in Smack project:
https://github.com/igniterealtime/Smack/pull/179
https://github.com/igniterealtime/Smack/pull/181

jitsi/jicofo,jitsi/jicofo,jitsi/jicofo,jitsi/jicofo