[openstreetmap/openstreetmap-website] Lock changesets during a diff upload (PR #6379)

[Andy Allan]

gravitystorm left a comment (openstreetmap/openstreetmap-website#6379)

I'm not very good with database locks, but I have two questions:

```ruby
changeset = Changeset.lock.find(params[:changeset_id])
```

Is this different from `changeset = Changeset.find(params[:changeset_id]).lock!` ? As far as I can tell from the docs, it's the same thing, but we use the `.lock!` variant elsewhere. If they are indeed identical, then I'd prefer to use the `.lock!` variant here, since that makes slightly easier to understand that it's the changeset being locked, and not e.g. the whole `Changeset` table.

Second question, in #6366 the "safe navigation operator" was used, e.g. `node.changeset&.lock!`. I found that surprising, since taking a lock is generally not optional! Is this deliberate, or is it a side-effect of taking the lock too early on e.g. should the lock be taken after the code that validates the changeset exists? Or is there something else there about locks that I don't understand?

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