Implement the RELAYD_ROTATE_PENDING relay daemon command

This command allows the sessiond to check if a rotation is complete
from the relayd point of view. There can be a significant delay
between the time the consumer has finished extracting the data from
the buffers and the time the relay has finished writing them on disk,
and we can only inform the user that the rotation is complete when all
the data is on disk. So the RELAYD_ROTATE_PENDING command is used to
poll the relayd after the consumer has finished extracting the data
until everything is on the relayd disk.

This command also takes care of streams that did not exist on the
consumer when the rotation started, or streams that appeared after the
last rotation started. The chunk_id field is used to distinguish those
cases.

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