Skeleton tracking fails to parse with more than one user

[shamlian]

As written, the skeleton-tracking bridge does not work when there is more than one user in the scene. This appears to be due to a dependence on a max buffer size of 28000 characters, which is exceeded once two people are tracked. A fix will affect both the server in Kinect2ServerService.cs and in the client in startBody.cpp.

[changliu6]

I'm also having this bug. Has anyone fixed it?

[psigen]

I haven't finished the fix for the client yet yet, but I did update k2_server master to send using JSON without a fixed size in the cleanup branch.

https://github.com/personalrobotics/k2_server/blob/cleanup/Kinect2Server/Kinect2ServerService.cs#L260

We still need to update k2_client to match.