MEDIUM: stats: include disabled proxies that hold active sessions to stats

MEDIUM: stats: include disabled proxies that hold active sessions to stats

After reloading HAProxy, the old process may still hold active sessions.
Currently there is no way to gather information, how many sessions such
a process still holds. This patch will not exclude disabled proxies from
stats output when they hold at least one active session. This will allow
sending `!@<PID> show stat` through a master socket to the disabled
process and have it returning its stats data.