Refactor the PageCacheMonitor interface to be able to probe much deeper.
Reimplement by-default disabled monitoring of pin/unpin.

Enabling monitoring of pin and unpin can now be done at any time, because the underlying implementation uses MethodHandles and SwitchPoints.

The hidden system property that can enable it by default still exists, but it has been renamed and moved.
Merge pull request #3383 from chrisvest/2.2-new-pagecachemonitor

Refactor the PageCacheMonitor interface to be able to probe much deeper.