Dashboard improvement: hide thread pool section if no thread pool data is in the stream

[JorritSalverda]

I've created a .net version of Hystrix which I'll soon open source, but as it mostly uses .net's async there doesn't seem to be a real need for thread pools. In the dashboard those sections keep displaying though with the Loading.. text. Perhaps it can be collapsed after a while if no metrics are coming in.

[mattrjacobs]

Yes, this seems like a good idea. It's pretty unlikely this makes it to the top of my priority list, however. See #1033 for a discussion on the dashboard roadmap.