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When multiple write operations conflict, we:

1. Try them in-order till one succeeds.
2. After the fact, re-order them such that the pending operations "happen" after
   the one that succeeds.
3. Return "success" for all the pending write operations for that key. This is
   acceptable because we're claiming that the operation that _actually_ succeeded
   happened "last" so it would have clobbered the other operations.

However, in the case of put, we still need to remove the temporary file that we
didn't end up moving into place.