(fs) introduce system property to choose WatchService on macOS

Summary

Control which implementation of WatchService is used on macOS.

Problem

The existing WatchService implementation on macOS has its limitations, but it is quite stable and has characteristics users have become accustomed to over the years. The new FSEvents-based implementation, while providing much better performance, introduces a risk of new bugs and certainly changes the expected characteristics, which may not be desirable for every user.

Solution

Instead of simply replacing the existing implementation with a new one, it is proposed to provide an opt-in new implementation with the help of a system property. This way, users not wishing to switch to the new implementation are not affected by the change and those willing to give the new implementation a try can do so with ease. Over time, the default may be re-considered in favor of the new FSEvents-based implementation.

Specification

FileSystem::newWatchService returns an object of class MacOSXWatchService if System.getProperty("sun.nio.fs.MacOSXWatchService.use") returns non-null value and an object of class PollingWatchService otherwise (the status quo).