Bug 1729: Make BlockingQueue events in EventDispatcherImpl volatile

Just like queue in PipelineOrchestratorImpl, events in EventDispatcherImpl
should be volatile since it is expected to be modified by different threads.

Ref gerrit in which Madhu fixes a similar issue in PipelineOrchestratorImpl:
https://git.opendaylight.org/gerrit/#/c/10843/

Change-Id: I923cb974788b0425e28af7823bf6cd23aeeb0f5f
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>