* When loading topics in bulk, only trigger state change callbacks after
all the topics have been loaded and we determine that state has actually
changed.
* State change callbacks are also only triggered when state has changed.
The use of JSON.stringify might raise some performance concerns here as this is a
performance sensitive codepath. However, I measured the time for each
`_setState` function call locally, by wrapping the function call with
`performance.now()`, and did not see any significant overhead.
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