Add BenchmarkGoroutinesWaiting

This benchmark tests the performance of the runtime when opening thousands of
goroutines simultaneously and leaving them waiting on a channel read. It seems
to expose a substantial performance regression in Go 1.1:

```
benchmark                                old ns/op    new ns/op    delta
BenchmarkGoroutinesWaiting                    3579       110215  +2979.49%
```

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