Bug 2551 - Statistics collection of random node fails when large number if switches...

Bug 2551 - Statistics collection of random node fails when large number if switches disconnects from controller.

When statistics manager is collecting stats and large number of switches disconnects from controller
,some time it causes OptimisticsLockFailedException while processing the existing multipart reply
present in dataStoreOperQueue. Exception occures because it tries to write data to the Node that
doesn't existing in the data store. While handling OptimisticsLockFailedException, statistics manager
flush existing data store related operation task from dataStoreOperQueue. This queue also holds Operational
task for cleaning up internal data of the disconnected nodes and because it flushes the queue, those
operational task don't get executed and statistics manager assumes that those nodes are yet connected
to the controller. When all the switches connects back with different mode (e.g previsouly Openflow13
but reconnects in OpenFlow10 mode), statistics collection of those node fails.

Change-Id: I50a44dfd20f90e3179bcd15bde67247da6565af7
Signed-off-by: Anil Vishnoi <vishnoianil@gmail.com>