Bug 1034 : Statistics request queue cleanup on node disconnection
MD-SAL Statistics-manager was not removing the statistics request entries from request scheduler queue. So even if
node is getting disconnected, request scheduler was sending the request down to the plugin and plugin was throwing
IllegalArgumentException. With this fix, whenever node get disconnected its respective NodeStatisticsHandler clean up
its pending statistics request from scheduler queue.

Change-Id: Ie70544b11f660531312c29ff89bb868761104282
Signed-off-by: Anil Vishnoi <avishnoi@in.ibm.com>
(cherry picked from commit 612c345a6df5f01d94c256560abd57eedd806a0d)