Empirical measurements indicate on the network file system we use
internally, using `FileStat.stat` on thousands of files is much
faster than using `FileStat.statSync`. (It can be slower for files
on a local SSD, however.)

Add a flag to `ProjectFileInvalidator.findInvalidated` to let it
use `FileStat.stat` instead of `FileStat.statSync` when scanning for
modified files.  This can be enabled by overriding `HotRunnerConfig`.

I considered creating a separate, asynchronous version of
`findInvalidated`, but that led to more code duplication than I
liked, and it would be harder to avoid drift between the versions.