Polling Priority (mostly done)

[FinboySlick]

I'm putting this in feature request rather than plugin because it requires a simple patch to spine (included).

The plugin lets you set a polling order. This can be important in very large (several thousand hosts) deployments when a significant portion of the network goes down. Timeouts, even if set low, tend to add up past the poller period and cause gaps in graphs for hosts that were still reachable. WISP would benefit from this by priorizing critical infrastructure and de priorizing less important (customer premise) devices.

I'd be keen to discuss how this could be made into a feature, which would likely mean changing cmd.php and modifying a few more tables. I only use spine as I expect most people who would benefit from this do. Consequently, the code in its current state only works with spine.