An object of this class is registered to be invoked in some predefined time intervals. This
checks the timeouts of callbacks stored in the SynapseCallbackReceiver and removes all expired
callbacks. Timeouts of the callbacks are stored as the time, not the duration. So that the
time or the interval of invoking this class does not affect the correctness of the timeouts,
although longer intervals would introduce larger error between the actual timeout and the
For each invocation this gets a time value to be compared against the timeouts of the callback
objects. This time is the System.currentTimeMillis() for Java 1.4 and System.nanoTime() for
Java 1.5 and later.