BUG-1120: add ExecutorServiceUtil

ExecutorServiceUtil encapsulates the utilities needed to implement a
blocking-queue based, growing thread pool executor service. This is
useful in both the controller and the openflow plugin.

Change-Id: I888e9bbd7732dd589ec153cfac67d6a2b032103d
Signed-off-by: Robert Varga <rovarga@cisco.com>