Speed up overzooming in tile-join (#147)

* Clip away entire features by bbox. Avoid unnecessary recompression.

* Move parent tile decoding in tile-join out of overzoom proper

* An ever-growing cache of parent tiles

* Limit the size of the cache

* Remove the current reader *before* checking if we can run the queue

* Clean up

* Add missing #include

* Add comment

* When the tile-join cache fills up, evict the least recently used

* Fix microsecond math

* Factoring out tile-join's cache for testing

* Add unit tests for tile-join cache

* Update changelog and version

overzoom.cpp

@@ -91,7 +91,7 @@ int main(int argc, char **argv) {
 		exit(EXIT_FAILURE);
 	}
 
-	std::string out = overzoom(tile, oz, ox, oy, nz, nx, ny, detail, buffer, keep);
+	std::string out = overzoom(tile, oz, ox, oy, nz, nx, ny, detail, buffer, keep, true);
 	fwrite(out.c_str(), sizeof(char), out.size(), f);
 	fclose(f);