swr: [rasterizer core] buckets fixes

swr: [rasterizer core] buckets fixes

1. Don't clear bucket descriptions to fix issues with sim level
   buckets getting out of sync.

2. Close out threadviz file descriptors in ClearThreads().

3. Skip buckets for jitter based buckets when multithreaded. We need
   thread local storage through llvm jit functions to be fixed before
   we can enable this.

4. Fix buckets StopCapture to correctly detect capture complete.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>