MINOR: stats: add 3 new output values for the per-server idle conn state

The servers have internal states describing the status of idle connections,
unfortunately these were not exported in the stats. This patch adds the 3
following gauges:

 - idle_conn_cur : Current number of unsafe idle connections
 - safe_conn_cur : Current number of safe idle connections
 - used_conn_cur : Current number of connections in use
2 files changed