Fix: relayd: per-pid live: no new metadata vs close

Fix: relayd: per-pid live: no new metadata vs close

When using lttng-live on a per-pid UST trace, the metadata stream is
closed after an application exits. The live viewer observes that the
stream has no more data when getting the metadata returns 0 bytes.

Internally in the relay daemon, it is assumed that a metadata stream
can be put (and thus removed) as soon as all the metadata has been sent
to the viewer, but this is not quite right. The viewer actually needs to
observe a 0-byte reply after receiving all the metadata in order to
gracefully perceive the metadata stream hang up.

Therefore, add a no_new_metadata_notified flag to the metadata stream
to track whether that 0-byte message has been sent to the viewer since
the last metadata content was sent, and postpone the reclamation of the
metadata stream until it is closed _and_ that 0-byte reply was sent to
the live viewer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I0a05332284299d62b832046e4f9d22b6029c3a3e
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>