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 publishes data from [Prosody's `internal` statistics][doc:statistics]
 into a PubSub node in [XEP-0039] format.

The node defaults to `stats` but can be changed with the option
`pubsub_stats_node`.

``` lua
Component "pubsub.example.com" "pubsub"
modules_enabled = {
    "pubsub_stats";
}
pubsub_stats_node = "statistics"
```