Adopting this older ticket to revisit ping-check behaviour in the light of more issues reported with PXE-boot clients:

Need to look at three things:

1. Configurable switch to not ping-check on returning clients
2. Make ping check threshold currently hard-coded at 60 seconds configurable
3. Determine if we are inappropriately pinging leases identified as reusable (i.e. younger than cache threshold)