Fixes a bug in the instance store where insert and bulkUpsert would
overwrite existing properties if there was a row there already. Now
it'll ignore any properties that are undefined.
The implementation is lifted directly from
`src/lib/db/client-applications-store.ts` (line 107 atm).
Additionally, I've renamed the `insert` method to `upsert` to make it
clearer what it does (and because we already have `bulkUpsert`). The
method seems to only be used in tests, anyway. I do not anticipate any
changes to be required in enterprise (I've checked).
## Discussion points:
This implementation uses `delete` to remove properties from the object.
Why didn't I do it some other way? Two main reasons:
1. We've had this implementation for 4 years in the client applications
store. If there were serious issues with it, we'd probably know by know.
(Probably.)
2. The only way I can think of without deleting, would be to use
`Object.fromEntries` and `Object.toEntries` and either map or reduce.
That'll double the amount of property iterations we'll need to do.
So naively, this strikes me as being more efficient. If you know better
solutions, I will of course be happy to take them. If not, I'd like to
leave this as is and then change it if we see that it's causing issues.
