Add session cleanup.

When finished running (either normally or through ctrl+c), benchmarks.py now
destroys the current session (if any), kills the session daemon if it started
it and deletes the temporary directory.
Add side-effects to internal loops

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Samuel Demers <samuel@samdemers.com>