Fix: iterator set_pos

Iterating over all the streams of the trace_collection was a mistake
since we could not identify the end of the streams (so we would always
save the position of the last event).

When we save the new position, we now iterate over each stream active in
a copy of the current heap to save their position. That way we don't
save the position for streams read entirely.

Fixes: #293
Fixes: #296
[ Edit by Mathieu Desnoyers: fix memory leaks on errors ]

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