Eliminate race conditions in floating association

This makes associating and disassociating floating ips atomic and
idempotent. This means multiple concurrent messages will not leave
behind iptables rules and concurrent request will not cause odd
failures.

Fixes bug 1092762 and bug 1092761.

Change-Id: Idbcad6c1d2a3d4881cf7180b848ed3844fac4054
Merge "Eliminate race conditions in floating association"