Provide a flag to disable file hash check

[anton-fomin]

https://github.com/gmethvin/directory-watcher does file hash check by default
to deduplicate change events. But that can increase the startup time on
large code base.

Fixes #110

[lightbend-cla-validator]

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[jtjeferreira]

@raboof could you have a look?

[jtjeferreira]

@mkurz since you are actively working on play issues and PRs, do you think you could help move this forward?

[mkurz]

Do I understand correctly that the file hash check mechanism is only relevant when running an app with ~run mode?
So it can safely be turned off when just using run (when a page reload triggers re-compilation and reloading of the app)?

[mkurz]

I think there is a mistake with the default.

[jtjeferreira]

Do I understand correctly that the file hash check mechanism is only relevant when running an app with ~run mode?
So it can safely be turned off when just using run (when a page reload triggers re-compilation and reloading of the app)?

I think so, however this PR does not change anything, it will just allow play or users to change it without making a copy of DefaultFileWatchService and then setting PlayKeys.fileWatchService

In our work project changing this saved around 5s when starting the app. See also a flamegraph where we saved around 5s:

before:

after:

The purple is from searching "watch" which you can see is now reduced...

[mkurz]

@jtjeferreira Curious, how large is your project? How many files? Sub-projects?

[jtjeferreira]

@jtjeferreira Curious, how large is your project? How many files? Sub-projects?

around 6000 scala files, 60 sub projects