Summary
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Define a new system property `"jdk.nio.file.WatchService.maxEventsPerPoll"` to set the maximum number of `java.nio.file.WatchEvent`s which a `java.nio.file.WatchService` can emit before dropping events and emitting an `OVERFLOW` event.

Problem
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There is presently a hard limit of 512 on the number of watch events which can be emitted before some events are discarded and an `OVERFLOW` event emitted. In some use cases this can become problematic, for example watching the contents of a very large repository.

Solution
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Add a system property `"jdk.nio.file.WatchService.maxEventsPerPoll"` to control the maximum number of events allowed before discarding events and emitting an `OVERFLOW` event. The default value of the new property is the same as the previous hard-coded event limit.

Specification
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    --- a/src/java.base/share/classes/java/nio/file/WatchService.java
    +++ b/src/java.base/share/classes/java/nio/file/WatchService.java
    @@ -98,6 +98,20 @@
      * it is not required that changes to files carried out on remote systems be
      * detected.
      *
    + * @implNote
    + * The JDK's {@code WatchService} implementations buffer up to 512 pending
    + * events for each registered watchable object. If this limit is exceeded,
    + * pending events are discarded and the special
    + * {@link StandardWatchEventKinds#OVERFLOW OVERFLOW} event is queued. This
    + * special event is the trigger to re-examine the state of the object, e.g.
    + * scan a watched directory to get an updated list of the files in the
    + * directory. The limit for the pending events can be changed from its default
    + * with the system property
    + * {@systemProperty jdk.nio.file.WatchService.maxEventsPerPoll}
    + * set to a value that parses as a positive integer. This may be useful in
    + * environments where there is a high volume of changes and where the impact
    + * of discarded events is high.
    + *
      * @since 1.7
      *
      * @see FileSystem#newWatchService