vtzero::convert_property_value inside worker

[mapsam]

The set_property method does some variant decoding in the main thread while constructing the final JSON object to return to node. This, in theory, could happen within the worker threads before being handled in the callback, and increase performance.

vtquery/src/vtquery.cpp

Lines 154 to 160 in 95ab347

	/// used to create the final v8 (JSON) object to return to the user
	void set_property(vtzero::property const& property,
	v8::Local<v8::Object>& properties_obj) {
	auto val = vtzero::convert_property_value<mapbox::feature::value, mapbox::vector_tile::detail::property_value_mapping>(property.value());
	mapbox::util::apply_visitor(property_value_visitor{properties_obj, std::string(property.key())}, val);
	}

cc @springmeyer @flippmoke

[springmeyer]

@mapsam, this change is implemented already in #73: see the properties_vector_materialized member of the ResultObject. It is REQUIRED in the case of #73 because the actual properties are no longer in scope after returning from the threadpool (because the buffer, in the gzip case, is not alive outside the threadpool). See https://github.com/mapbox/vtquery/pull/73/files#diff-d293222b917c4d9edf75c1288538554fR378 for more details.

[mapsam]

Oh awesome @springmeyer - thanks for pointing that out! 0.2.0 was just released, so perhaps we can work on this going out in 0.3.0?

[springmeyer]

Done in #95