Use a single Timer instead of two threads

It turns out that we really want to have the aging and collection bound
to a common timeline, so let's combine them. This will give us the
opportunity to perform mark&sweep in future.

Furthermore we use the threads only for periodic tasks, so let's convert
to a Timer with TimerTasks. This will allow us more flexibility going
forward, for example allowing NodeStatisticsHandlers to manage how often
they wish to collect individual statistics.

Change-Id: Ie2cd0e4e7391e1175a8927773f85278b80f16c48
Signed-off-by: Robert Varga <rovarga@cisco.com>