MINOR: proxy: extend "show servers conn" output

MINOR: proxy: extend "show servers conn" output

CLI command "show servers conn" is used as a debugging tool to monitor
the number of connections per server. This patch extends its output by
adding the content of two server counters.

<served> is the first added column. It represents the number of active
streams on a server. <curr_sess_idle_conns> is the second added column.
This is a recently added value which account private idle connections
referencing a server.