Demo does not work in Microsoft Edge 42.17134.1.0

Reported by @tiloio originally at https://github.com/Kinto/kinto/issues/1852. On synchronization, we get `list() DataError`, and on adding a task, we get a `DOMException` with message `DataError`.

Because `DataError` does not occur in our codebase, this is probably a quirk of the Edge IndexedDB implementation. [caniuse](https://caniuse.com/#search=indexed) seems to think that Edge supports most of the IndexedDB v1 spec except for inside "blob web workers", whatever those are. However, `getAll` is not supported.

@tiloio, any idea if the kinto demo works with version v12.1.1 or earlier? We recently made some changes to use `getAll` in some places -- maybe we need to add checks around that, or maybe the problem is something deeper like https://github.com/Kinto/kinto.js/blob/master/src/adapters/IDB.js#L50.

Reproduce synchronization and task creation in the demo with Microsoft Edge 42.17134.1.0, then inspect src/adapters/IDB.js around the referenced line and the recent getAll changes. Compare behavior with Kinto.js v12.1.1 or earlier; done means identifying the Edge IndexedDB cause and confirming the demo works or documenting the remaining limitation.

Written by the indexing model from the issue text.