BUG/MINOR: stats-file: manipulate shm-stats-file heartbeat using unsigned int

BUG/MINOR: stats-file: manipulate shm-stats-file heartbeat using unsigned int

shm-stats-file heartbeat is derived from now_ms with an extra time added
to it, thus it should be handled using the same time as now_ms is.

Until now, we used to handle heartbeat using signed integer. This was not
found to cause severe harm but it could result in improper handling due
to early wrapping because of signedness for instance, so let's better fix
that before it becomes a real issue.

It should be backported in 3.3