Before enabling strong ownership features in the iOS wallet, ensure we handle some backcompat cases

[kimdhamilton]

This issue describes the originally reported problem: blockchain-certificates/BlockcertsFramework-iOS#33

I pushed commits to the framework and to this repo (e6462ac) that addressed the immediate concern of initializing keychains properly.

Part of the change was to future-proof for wallets that have already handed out keys, as follows:

Saves index state in under a new name (even for existing implementations), which will enable existing wallets (that have already used keys from the former derivation) to know the max index that was used. This will allow us recover ownership, for example when the wallet supports features such as proving ownership of keys.

This backcompat is required only for Blockcerts wallets where public addresses were generated before this fix (which we can detect by presence of a certain key in the user defaults).