Fix: trace-collection: trace clock use after free

The trace collection should copy the trace clock object rather
than take a reference to the first trace's trace clock, because
it may be freed when the trace is removed (e.g. application going
away in per-pid live tracing).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>