Should support a (basic) resource pool? #76

In RobotLocomotion/drake#13971, @RussTedrake found that having HTML JS clients re-download the IIWA meshes every time for each new pydrake...MeshcatVisualizer instance is rather slow.

He was able to work around it by #75, which relies on using the same existing object path.

In an ideal path, there would be a way to (naively) cache the data objects (perhaps on the sending client side?) based on byte comparison.

Then you get the same benefit; additionally, if someone were to visualize a large scene (e.g. 20 IIWAs in the same setup), the websocket only has to receive the meshes once (rather than 20 times).

\cc @gizatt