consumerd: tag metadata channel as being part of a live session

consumerd: tag metadata channel as being part of a live session

metadata channels that are part of a live session must be handled
differently than when they are part of non-live sessions since
complete "metadata units" must be accumulated before they are
forwarded to a relay daemon.

This allows a follow-up fix to use this information since the
live_timer_interval of a metadata channel is always 0.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I53db4bc717b149ed20e0309531db6f0241e873e1