SPY-172: Wakeup the selector if idle.

Motivation
----------
Waking the selector up from time to time and providing implementations a
chance to run certain checks helps to improve robustness in certain
situations.

Modifications
-------------
The wakeup time is configurable through a system property, but is low
impact even if set to a smaller value. If the added queue is empty (which
means the selector has been woken up but no op has been added) a custom
method is called where implementations can run custom code like idle polls.

Result
------
Better handling in idle situations.

Change-Id: I43ea722b8a4fc28be4f997674ea85f73f2c66a50
Reviewed-on: http://review.couchbase.org/37725
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>