MEDIUM: stats-file: processes share the same clock source from shm-stats-file

MEDIUM: stats-file: processes share the same clock source from shm-stats-file

The use of the "shm-stats-file" directive now implies that all processes
using the same file now share a common clock source, this is required
for consistency regarding time-related operations.

The clock source is stored in the shm stats file header.
When the directive is set, all processes share the same clock
(global_now_ms and global_now_ns both point to variables in the map),
this is required for time-based counters such as freq counters to work
consistently. Since all processes manipulate global clock with atomic
operations exclusively during runtime, and don't systematically relies
on it (thanks to local now_ms and now_ns), it is pretty much transparent.