Fix: mmap trace read the stream_id from the first packet

Fix: mmap trace read the stream_id from the first packet

We were hardcoding the stream_id as 0 for mmap traces, causing a
problem for live traces with multiple channels.
A fix enters in lttng-tools v2.4.2 to send the CTF stream id in the beacon
packets, but until then, we cannot support traces with multiple CTF
streams (LTTng channels).

Fixes: #811
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>