Implement the relayd live features

The live-reading feature allows a user to read the trace while it is
being recorded. In order to work, this feature requires the trace to be
streamed to lttng-relayd. Then, a viewer can connect to the relayd, list
the sessions, attach to one, and start reading the data. The
live-reading protocol, enforces the viewer to read all the metadata
before trying to read the trace, and checks all the streams for new
activity at a user-defined rate (configured by the new --live parameter
during the session creation).

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>