API changes

[tannerpreiss]

Original issue said:

Changes for local persistence:

• Add a "pending" field (boolean) to CSValue.
• Make the aclid in CSValue optional.

Misc API changes (all SDKs)

• Remove the "providerData" field from CSAuthData
• Add Key.makeChild to create a key with UUID

iOS only:

• Add variants of write, delete with no completion handler

Updates:

Rather than pending, a negatively biased name, I'd like to use the property name stable, which is positively biased.

I think we should use Key.child() (with no parameter) as the method to create a child key with UUID, rather than Key.makeChild().