In this commit, we add a new package-level function to emulate the
existing Batch interface for bbolt. We do this via a new super-set
interface which is then checked against in the main implementation of
the Batch method.
In this commit, we create a new module for `walletdb` in order to allow
external projects to use the project without also having to depend on
the entirety of `btcwallet`. With this commit, projects that depend on
`walletdb` will benefit from a slimmer set of additional dependencies.
