Significant performance improvements, URDF refactor

[brentyi]

This builds on #63, so the diff will be messy until that's merged.

A handful of changes:

• Added viser.extras.ViserUrdf, which is a light wrapper around the relevant yourdfpy + viser APIs for visualizing URDF files.
• Significant performance improvements when we have a lot of messages. The largest change here is a TCP-inspired windowing implementation, which has a big impact on client-side deserialization overhead. This helps a lot for things like real-time visualization of camera pose optimization or high-dimensional robot states (for many robots).
• More robust .atomic() implementation. The previous one had some race conditions related to culling for redundant messages, and would fail for high message volumes.