Make sure "starttls_required" can't be bypassed

Don't allow clients to circumvent the "starttls_required" option by
enabling XMPP stream compression.

Inspired by weiss/ejabberd@7bdc1151b11d26d33649c5cce2817b74a4f231a8.
Merge pull request #304 from esl/starttls_bypass

Make sure "starttls_required" can't be bypassed