Bug 9066:Use Single Transaction for DNAT Flow Install and Remove

Bug 9066:Use Single Transaction for DNAT Flow Install and Remove

Problem Description:
===================
Use one transaction for DNAT flow install and use another single
transaction for Removing the flows from DNAT switch.

As of now each DNAT (Floating-ip) flow installation, it used separate
transaction object for each flow installation.

Solution:
==========
Used single write transaction object for all DNAT(Floating-IP) flows installation on
the switch and used other
write transaction object for removing all DNAT flows on the switch.

Change-Id: Idf8f0c9ce36b20b14ede9a423c574ee21f36d02e
Signed-off-by: karthikeyan <karthikeyan.k@altencalsoftlabs.com>