Cannot access AsyncServletProperties.Threads fields because it is a package private class

[yehor-masalitin]

Describe the bug
I would like to get access to config from Threads class, but I can't at the moment as it is a package-private class.
I think it would be great to have it public, so we can use it as a part of any other custom settings

To Reproduce
Try to use AsyncServletProperties.Threads in any package which is not where it is located

Expected behavior
AsyncServletProperties.Threads should be available in all packages

Screenshots

[yehormasalitin]

Pull request: #749