Relay rotate pending command

When a session rotation completes and the session is configured to send
its traces to a relay, we have to poll the relay to know when all the
chunk's data are written on its disk. To do that, we define a timer in
the sessiond and arm it when the rotation is complete. When the rotation
is complete on the relay, we clear the "rotate_pending" flag in the
session and the client can access the chunk safely.

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