Fix the regression introduced by 9af79cf51a where updateAttributes
and replaceAttributes was incorrectly handling the response returned
by the database.
This commit restores the old behaviour where `context.data` is updated
only by a connector-provided function and reverts an incorrect change
of a test file made as part of the faulty fix.
Besides the fixes, this patch also renames `data` to `dbResponse` and
add comments explaining the structure of the response object for more
clarity.
9bd7f8d02f