QServer metrics: use ISOServer's activeConnections

[barspi]

QServer was producing the metric:
ISOSERVER_CONNECTION_COUNT("jpos.server.connections", "Incoming active connections")

with a unit of THREADS (prometheus full name jpos_server_connections_threads{....})

This was actually using ISOServer's getConnectionCount(), which returns the historical total count (ever increasing, unless resetCounters() is explicitly called by someone) and not the count of currently active connextions.

The active connection count is both more useful and matches the HELP description of "Incoming active connections".

Also, the suffix/units of THREADS wasn't conveying useful info, and would leak implementation details.
We noticed that ChannelAdaptor is using BaseUnits.SESSIONS.

This PR attempts to fix both issues:

• returns the number of active connections
• It returns a metric with _sessions suffix, to align it with ChannelAdaptor semantics.

An example:

# HELP jpos_server_connections_sessions Incoming active connections
# TYPE jpos_server_connections_sessions gauge
jpos_server_connections_sessions{name="my-server",type="server"} 1.0

NOTE: some current Grafana dashboards may need to be adapted to the new name.