Addon-docs: Ensure we don't clobber multiple source container state updates

[tmeasday]

Issue: As reported by @shilman

http://localhost:9011/?path=/docs/addons-docs-props--arg-types in Official Storybook

What I did

Ensure we don't clobber state if multiple events are received before a re-render finishes

How to test

• Is this testable with Jest or Chromatic screenshots?

I'm not sure if we have the ability to test this outside of e2e?

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[shilman]

LGTM and tested! ✅

@tmeasday My only concern is that AFAIK setSources(current) will trigger a re-render, so this might result in extra renders?

[tmeasday]

@shilman

(a) when we return current react will not re-render, that's part of the contract of useState.
(b) we actually re-render less than before, which is why this bug happened, on the plus side.

[tmeasday]

PS the code doesn't quite make sense to me (why do we both check for equality on (newItem !== sources[id]) and using deepEqual?) but I tried to change it semantically as little as possible.