fix(storage): array cursor iterator should return stats of all observed cursors

[GeorgeMac]

Closes #15732

1. Adds a failing test which demonstrates the incorrect reporting of stats from the arrayCursorIterator.Stats() method.
2. Updates arrayCursorIterator.Stats() to accumulate all the cursor field stats values into a single response.

@stuartcarnie showed me that the builldXArrayCursor methods actually retain previously read stats and accumlate them on calls to reset which is how all read stats are accumulated in the end.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[GeorgeMac]

I just ran this with our signed integer test and it failed.
So I think perhaps the accumulation of all cursors was the desired effect. I will dig further into this assumption.

[stuartcarnie]

Great stuff, @GeorgeMac – thanks for uncovering and fixing!

[GeorgeMac]

Cheers @stuartcarnie and @benbjohnson 👍

[e-dard]

Thanks @GeorgeMac. We were lucky to uncover this bug!