Command metadata regenerate

Command to regenerate the metadata file when the session is running.
This allows the user to regenerate the metadata after a major NTP
correction and that way update the clock offset from epoch in the
metadata.

Works for kernel and UST per-uid session local or remote (not live).

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>