mod_muc_markers: replace configurable multi-marker tracking with better system Now tracks a single marker, but automatically applies rules from the XEP about "higher" markers implying "lower" markers - i.e. "displayed" implies "received". Still, just a single marker is tracked and synthesized at all times (the one configured with muc_marker_name).

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.