Reduce memory consumption from attribute accumulation (#290)

* Progress on plumbing a string pool for full_keys through

* More plumbing for key_pool

* Don't keep features with identical locations as multiplier features

* Revert "Don't keep features with identical locations as multiplier features"

This reverts commit 413f0c8024.

* Adjust calculated maxzoom to account for duplicate feature locations

* Update changelog and version

* Add a test affected by the maxzoom change with duplicate locations

* Round the drop rate a little for cross-platform test consistency

tests/ne_110m_populated_places/out/--smallest-maximum-zoom-guess_2_-Bf20_-rf20_-pb.json

@@ -10,7 +10,7 @@
 "minzoom": "0",
 "name": "tests/ne_110m_populated_places/out/--smallest-maximum-zoom-guess_2_-Bf20_-rf20_-pb.json.check.mbtiles",
 "strategies": "[{\"dropped_by_rate\":237},{\"dropped_by_rate\":229},{}]",
-"tippecanoe_decisions": "{\"basezoom\":2,\"droprate\":6.599999999999999,\"retain_points_multiplier\":1}",
+"tippecanoe_decisions": "{\"basezoom\":2,\"droprate\":6.6,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/ne_110m_populated_places/out/--smallest-maximum-zoom-guess_3_-rp.json

@@ -10,7 +10,7 @@
 "minzoom": "0",
 "name": "tests/ne_110m_populated_places/out/--smallest-maximum-zoom-guess_3_-rp.json.check.mbtiles",
 "strategies": "[{\"dropped_by_rate\":230},{\"dropped_by_rate\":231},{\"dropped_by_rate\":160},{}]",
-"tippecanoe_decisions": "{\"basezoom\":3,\"droprate\":2.6582825467008166,\"retain_points_multiplier\":1}",
+"tippecanoe_decisions": "{\"basezoom\":3,\"droprate\":2.65828,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [