In order to correctly display the number of connected clients in our
monitor dashboard, we need to track those connections that are
actually connected to clients, not considering connections from
persistent sessions that are disconnected. Today, the
`connections.count` that is displayed in the dashboards considers
those disconnected persistent sessions as well.
Fixed bugs:
- Priority queue lack of a `len + 1` logic in `in/2`
Changed behaviors:
- Topics not found in priority table (from config) will be treated with default priority,
instead of hasing topic name to a priority number.
- Default priority is now configurable (it was always lower than all configured priorities)
- The dropped message due to reaching `max_len` is now returned from `in/2`,
so the queue owner (`in/2` caller) can perform autopsy on it
Thales Macedo Garitezi