Notifications not properly cleaned up when more than 1 exists

[tbronson]

Whenever more than 1 Notification is issued, only the last issued one will be properly cleaned and removed from the notification list along with its associated watcher in the _notification_watchers dict, due to timeout or user interaction closing it.

Calling clear() does not cleanup the watchers.

[tbronson]

Debugging on the javascript side, it seems that the syncing of the notifications array is causing a copy operation on the JS-side, which I'm guessing is creating new Notification objects for any already existing in the array (you can see the all the ids change each time you issue a new notification), while all the watchers and timeout callbacks are tied to the old orphaned Notifications.

I'm not savvy enough yet with Bokeh/Panel internals or coding in javascript to know the best way to fix this issue.