IAcceptResource to maintain Occupance count

[anvilvapre]

ClosestProc in OpenRA.Mods.Common/Traits/Harvester.cs uses quite an expensive query to calculate how much harvesters are already associated with a IAcceptResources trait. It queries all harvesters to see if they are linked to IAcceptResources trait - for each trait.

An optiomization would be to just maintain an Occupancy count per IAcceptResources. Harvesters would have to increment and decrement this value.

The value indicates how many harvesters are associated with the trait. Allowing the spreading of harvesters over traits/refineries.

Relevant whenever a harvester tries to find a new refinery.

[PunkPun]

That's part of the plan with the docking refactor. I'm waiting for a few PR's to be merged before I get to optimising it