Fix: relayd: make viewer streams consider metadata sent

The metadata stream does not use prev seq, and is therefore not sent to
viewers if we depend on prev seq. Use the metadata_received field
instead to achieve the same purpose: if a viewer try to attach to a
session that has not received metadata yet, it will get and error
(metadata stream cannot be found when attaching).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>