XKeyID (WalletID, AccountID) should be a property of the XKey

[coolaj86]

While creating the CoinJoin wallet I'm finding that I often have to pass in both walletKey and walletId, or accountKey and accountId, etc in order to cache the key to prevent generating it - since CoinJoin walks the usage rather than the index, and thousands of keys need to be generated quickly.

This creates the opportunity to accidentally pass in a wrong string or otherwise have bad management tracking the two variables.

Instead, we should generate the ID when we generate the XKey, and make it available as a property of the key.