Replace 'upsert' option with 'conflict' option.
Update test cases to reflect new conflict option.
Add tests for conflict=error and conflict=somearbitrarystring cases.
Use enum class instead of straight enum.
Reflect that serialization for this has changed & is network only.
Describe conflict=error option in protobuf file
Merge conflict resolution selection, issue #1838.

This was reviewed by @mlucy in review 1758.