Implement the RELAYD_ROTATE_STREAM relay daemon command

Support for the RELAYD_ROTATE_STREAM command on the relay. This
command informs the relay that the current stream must rotate after it
has written the data and index for the net_seq_num passed. After each
data and index written on disk we check if it is time to rotate (in
case it was in flight when the rotate command was received). On the
other hand, if too much data has been written when we receive the
rotate command, we move the excess data to a new tracefile and
truncate the current one.

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