[Observium] High latency poller performance

Hi All,

I've been testing some improvements to the Poller when deployed in high latency situations (polling hosts on different continents, for example).

It involved some SNMP tweaks to pull more OIDs per round-trip, but has varying support in different OSes, so we're making it a hardcoded per-OS attribute. It seems to work particularly well on Cisco IOS, IOS-XR and Arista devices.

http://alpha.memetic.org/~adama/snaps/fwy8vmmvii.png

Shows the performance gains on a selection of Cisco IOS devices.

I'll be committing the changes later today, disabled as default.

If anyone wants to test them for us before we enable them as default, give us a shout.

adam.