Rate-limit wifi and cell measure ingress in submit task, close #97

Another attempt at limiting wifi measurements to something reasonable; also limits cell measurements, for completeness sake.

I also added 3 new heka metrics here to track dropped measurements: one for cell measures we drop due to being malformed and one each for drops caused by this rate-limit overflow. Picked metric names out of the air, might want to change them.

I'm also not really happy about the hardcoded max_measures_per_cell default but I can't think of anywhere nicer to put it off hand. If you have a preference I'll follow it.