Cleanup: rotation-thread: enforce conding standard following fix

A fix introducing
rotate_notification_channel_subscription_change_eventfd didn't follow
the current coding standard so as to make it easier to backport to the
stable branches.

Clean-up the affected code to follow the current standard:
  - Replace the use of a raw eventfd to use the eventfd utility,
  - Subscribe and unsubscribe functions made use of global variables to
    communicate with the rotation thread: replace that with the
    rotation_thread class to centralize the interface,
  - Make the code using eventfd exception-safe (automatic memory
    management, use of various RAII utils),
  - Replacement of non-null pointers by references.

Change-Id: I7e363e21b829fd0939a336aca2570fdbcc346967
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>