[scudo] Add a Timer class to assist performance measurement

[scudo] Add a Timer class to assist performance measurement

Add Timer and TimingManager which provide convenient way to meause the
execution time of code snippets. The output looks like,

```
-- Average Operation Time -- -- Name (# of Calls) --
          1747.2(ns)            popBatch (59)
            92.3(ns)            popBatchImpl (73)
           101.6(ns)              EmptyBatchProcess (5)
          2587.0(ns)            pushBlocksImpl (13)
```

Note that `EmptyBatchProcess` is nested under the timer `popBatchImpl`.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D143626