Went poking around looking for what might be causing the reported problems where replay eventually becomes laggy

Nothing significant discovered yet but when looking at the profiles of the page deduplicateSnapshots was drowning out other function calls.

We call deduplicateSnapshots many times and merge existing snapshots with any new ones, then stringify each so that we can identify and remove duplicates

We can store the calculated key so that we only do the potentially expensive calculation once per item

We could move the hash into the logic's cache so that we don't need to reprocess existing items - but that's not a last thing in the day fix

This is a quick patch to process items less. After which deduplicateSnapshots doesn't show as a significant function call in page profiles