The JVM doesn't always do a good job of telling JNI modules that the JVM
is going away, which results in a crash in the JavaGlobal and/or
JavaWeakGlobal destructors as they try to delete the associated references
after the JVM has already gone away.
To protect against this, the Notifier now has a static variable that's set
when the Notifier instance (a singleton) is destroyed. This is used by
JavaGlobal and JavaWeakGlobal to detect when a process exit is in process.
Also only perform immediate notification to the callback actually
requesting the notification, not all existing callbacks.
Offset returned uids by 1 so uid=0 can be used to indicate immediate
notification.
On the callback function, is_new indicates the value is newly added.
On adding a callback function, immediate_notify indicates the callback
should be called once (with is_new=true) for each matching entry that
already exists.
