Fix: consumer should await for initial streams

lttng-sessiond need to let the consumer know how many streams are sent
initially, so that for very short traces (short-lived apps, short kernel
trace), the consumerd don't run into the scenario where it deletes the
channel when there are still pending streams to receive for this
channel.

Fixes #355

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