Disallow observing on transaction for onupgradeneeded

[dmurph]

I'd like to disallow observing on the transaction given to the onupgradeneeded callback for the following reasons:

1. This would observe ALL object stores, which could be a big performance hit depending on the implementation.
2. This would be the easiest copy-paste way to add observers, which (based on past usages of things) would get copy-pasted from stackoverflow or elsewhere, making the least-performant observer the default.
3. The user can instead just add the observer in onsuccess, which they would have to do anyways if there isn't an upgrade event (where it's just openning the db).

Another option offered by @pwnall is to add a sub-scoping option where in the .observe call, the user can specifically specify a sub-scope of object stores (where they would need to be also in that transaction). Then if one wanted to observe the onupgradeneeded transaction (which has a scope of all object stores), the user could specify only the ones they want.

I think this adds too much complexity, especially because the user would want to handle the non-upgrade-needed case anyways where they read the initial state and add an observer, which should solve this issue.

@bevis-tseng, @sicking, @inexorabletash, @pwnall: thoughts?

[dmurph]

I'm going to change the spec where this is disallowed, please re-open if you feel otherwise.

[dmurph]

Also @cmumford