Implement ring buffer periodic buffer switch timer

Implement ring buffer periodic buffer switch timer

The tricky part was the teardown. It must not race against neither
pending signals nor in-flight signal handler execution, otherwise the
timer handler could access freed channel data. Use a dedicated thread to
handle those signals, with a synchronization point between each handler
execution.

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