Performance-related improvements

I am currently working on resolving some performance issues in jdaviz. As part of this, I've identified the following things we could improve:

* We shouldn't notify if a value is set but hasn't changed (happens in jdaviz when setting .visible) - https://github.com/glue-viz/echo/pull/46
* ``CallbackContainer`` should be smart about avoiding duplicates as in some cases there are six callbacks to the exact same method - https://github.com/glue-viz/echo/pull/48
* We should think about how ``delay_callback`` works when called on an item which is a ``CallbackList`` - with jdaviz if I disable the notification of the CallbackList if any of the components change, things speed up by a factor of 3
* A way to delay all callbacks for example if I need to loop through all layer states and set all visibles to False