move semantics for the buffer inside mapnik.VectorTile

[springmeyer]

The getData() call copies the entire, uncompressed, vector tile buffer. This can be expensive.

We should add support for a takeData() or release() function which returns a node.Buffer which owns the underlying memory (the original mapnik.VectorTile object would then be empty).

This will allow for much more efficient passing of data to other node C++ addons like https://github.com/mapbox/vtcomposite and https://github.com/mapbox/vtquery.

/cc @mapbox/core-tech @flippmoke

refs mapbox/node-cpp-skel#69 and mapbox/node-cpp-skel#67

[springmeyer]

I've decided not to move ahead with this. I was assuming that the buffer type stored on the node-mapnik object and would be easy to change into a std::unique_ptr<std::string> to allow passing ownership around. But, it turns out the underlying type of the buffer is a std::string very deep inside mapnik-vector-tile. I'm therefore reluctant to change this given the greater time expensive to modifying both libraries. So, I'm punting on this and will plan to unlock the efficiency need I have by completely avoiding node-mapnik usage over at mapbox/tilelive-vector#155

[flippmoke]

Took a shot at this with #889 and mapbox/mapnik-vector-tile#285

[flippmoke]

Released in 4.0.1