Dynamic config subscriptions

[dnr]

What changed?

• Add Subscribe method to dynamic config settings.
• Support efficient notifications from FileBasedClient (old+new files are diffed and only changed keys are checked for subscriptions).
• Move benchmark code to new file.
• Change constrained defaults to pointer to slice since it's hardly ever used.

Why?

So that components can get notified on dynamic config changes instead of polling.

How did you test it?

unit tests