batch notifications

[jba]

The server should batch notifications to the client, to avoid a flurry of messages when many tools, etc. are added at once.

This actually happens in practice: see #73 (comment).

All AddTool notifications are the same (the params are empty), so we can count them and send one when the count reaches a threshold.
The same is true for all the other add and remove notifications.

This should happen in Server.changeAndNotify.

Also, we should asynchronously and periodically send notifications for any non-zero counts. The period should be fairly short (50 ms?) So that single calls to AddTool, etc. are notified promptly.

[findleyr]

Thanks for filing. @markus-kusano this might be a narrow one for you to pick up. Otherwise, I will do it.

[jba]

I'm going to take a shot. I'll relinquish if I can't finish it.