Performance-related improvements

[astrofrog]

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