Reduce memory consumption during tiling (#177)

* Drop duplicate geometries sooner in coalescing-as-needed

* Concatenate geometries before partial cleaning

* Spend less time with two feature representations in memory

* Further reduce in-memory duplication

* Remove another copy

* Don't keep duplicates in memory while coalescing

* Also clear out the mvt layer once it is no longer needed

* Update changelog

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 2.39.0
+
+* Reduce memory usage during tiling
+
 # 2.38.0
 
 * Tolerate polygon rings with insuffiently many points in input