Move metadata creation into lttng-sessiond and lttng-consumed

Move metadata creation into lttng-sessiond and lttng-consumed

The metadata is completely generated in the session daemon and pushed to
the consumer which writes the full string to a ustctl channel object. On
each event/channel notification from the tracer, the metadata is
generated on the spot and kept in the UST registry of the session
daemon.

On stop and destroy command, the metadata is pushed to the consumer. On
application unregistration as well, the metadata is handled.

Needs to be use in locked-step with LTTng-UST:
"Move metadata creation into lttng-sessiond and lttng-consumed"

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>