Rarely dhcp agent rpc call get_active_networks_info() can interleave
with network scheduling initiated by create.port.end notification.
In this case scheduling raises and port creation returns 500.
Need to synchronize on DhcpNetworkBindings table.
Closes-Bug: #1276552
Change-Id: I52d94a40772a99c7032dba15b200bf0f21362f93