Report self time and total time in performance summary

(result of a conversation offline with @angocke)

When looking at the performance summary, you often see numbers like

```
2274870 ms ResolveProjectReferences 71 calls
200719 ms CoreCompile 71 calls
```

It's quite surprising to see ResolveProjectReferences take so long. It's happening because time spent waiting on the references to build from within one project (while the node is yielded and hopefully doing other useful work) is attributed to the MSBuild task and to RPR.

We should consider adding a "self time" column to the output to report time spent in tasks and targets _excluding time the node is yielded_. That would help understand where the time is actually being spent.