popularity bands

[missinglink]

As discussed in pelias/openstreetmap#493 (comment), this draft PR ensures that documents on the postalcode layer receive a minimum popularity value.

I'm opening this up for testing now, I think (if it's successful) that we will probably want to make the MIN_POPULARITY_MAP structure configurable via pelias/config.

note: I've selected an initial value of 9000 for testing postalcodes, this is just a guess based on the values in https://github.com/pelias/openstreetmap/blob/master/stream/popularity_mapper.js and may need to be refined, pending testing.

[missinglink]

opening for review, needs a quick look-over before merging, add comments about later opportunities for configuration.

[missinglink]

testing showed a significant improvement.
note: this improvement is subjective, as it results in postcodes having more weight compared to venues than before.

we are open to extending/tuning this functionality and/or making it configurable via pelias/config at a later date, we will address those concerns if/when they arise in the future.