Fix wrong calculation for Uptime_sec
Fix wrong calculation for Uptime_sec

We used to sum the values of 'Uptime_sec' across all processes which
caused incorrect uptime value in multi-process mode for (nbproc > 1).

We fix this by calculating the median.
Closes #6

unixsurfer/haproxystats,unixsurfer/haproxystats,unixsurfer/haproxystats