Fix race condition on processing DVR floating IPs

Fip namespace and agent gateway port can be shared by multiple dvr routers.
This change uses a set as the control variable for these shared resources
and ensures that Test and Set operation on the control variable are
performed atomically so that race conditions do not occur among
multiple threads processing floating IPs.
Limitation: The scope of this change is limited to addressing the race
condition described in the bug report. It may not address other issues
such as pre-existing issue with handling of DVR floatingips on agent
restart.

closes-bug: #1381238

cherry-picked from 9902400039018d77aa3034147cfb24ca4b2353f6
Change-Id: I6dc2b7bad6e8ddbaa86c1f7a1e2028aeacc3afef
Merge "Fix race condition on processing DVR floating IPs" into stable/juno