Turn document update mode atoms into defines

We have made some mistakes there in the past mis-typing those. For example in
https://github.com/apache/couchdb/pull/3934, so as a preliminary PR to fix
https://github.com/apache/couchdb/issues/4121, where we'd have to again special
case logic based on the update type, let's switch them to be defines so the
compiler double-check the values for us.
20 files changed