Fix: join consumer timer thread

Detaching the timer thread has the unfortunate side-effect of letting
the health management data structures be freed by main() while the timer
thread may still be using them (if, e.g., main() exits quickly).

Overcome this situation by tearing down and joining the timer thread.

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