Quieter polling objects, updated timing semantics

The NNPollingObject superclass no longer logs anything, it is up to each subclass to perform any timing or liveness checks that they may need.
The poll timing has also changed. No longer is the time spent performing the last iteration credited to the next interval, each iteration begins a full interval after the previous completed. In addition, notification dispatch blocks execution of the caller, yielding the worker queue to other jobs on the runloop until the main thread has finished processing the notification.