pouchdb-upsert: Enable strictFunctionTypes and make the type of the parameter to UpsertDiffCallback more useful.

[mattmccutchen]

Addresses https://stackoverflow.com/q/53275861 .

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[typescript-bot]

@mattmccutchen Thank you for submitting this PR!

🔔 @keithdmoore @hotforfeature @apolkingg8 @zamb3zi - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

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[typescript-bot]

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