Keep read-only copies of fields from the channel to the stream

In the consumer, we sometimes need to read the channel pathname or
tracefile_size from a stream, but we cannot always access those values
safely. So we now keep a copy of those values when we add or allocate
streams.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>