Fix: unhandled prev_seq initial value

The previous sequence number of a stream is initialized to -1ULL
and comparing the current sequence number against it to perform
a rotation will yield unexepected results.

The assumption that the previous sequence number is less than
the current one is assert()'ed on elsewhere.

This case triggers whenever a rotation is performed before the
relay daemon has received a packet for a given stream.

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