add arp_sleep to throttle gratuitous arp

Hi,

I created this small patch to resolve an issue we are having with keepalived.
On servers with alot of virtual IP's, we have issues that our switches throttle the gratuitous arp packages.

This can't be changed on switch level (hardcoded), so we had to find some 'workaround'/'fix' for this.
So I implemented the following patch.

The patch makes you able to configure an 'arp_sleep' variable.
This will cause a small usleep between the arp's, which causes to resolve the throttling.

If you don't want the feature, you can just not set it (default = 0), and it won't affect anything.

The change also contains a seconds patch which is a rewrite of the code to a threaded version based on the comments in https://github.com/acassen/keepalived/pull/108.

Thanks!
Jean-Louis