OFplugin has a strong-referenced unbounded cache

Durting performance testing we have found that OF plugin keeps a strong-referenced cache, which is only boiund by time and dominated by strong references.

This means that as throughput goes up, so will the memory pressure.

Make the depth of the cache configurable and make it use softvalues, so that it a) has an upper bound on memory retained irrespective of throughput 2) reacts to memory pressure by sacrificing this information instead of running out of memory.