In this commit, we create a new package `kvdb`, which is meant to serve
as the basis for any future database abstractions within `lnd`. Rather
than directly use the `walletdb` package (which we base off of), we
instead use a series of type-aliases to re-type the fundamental
types/interfaces of the `walletdb` package. This lets us type
`kvdb.RwTx` instead of `walletdb.ReadWriteTransaction` everywhere.
Additionally, our usage of type-aliases is also intended to create an
easy pathway in the future wherein we can gradually re-defined or
re-implement these types to wean off of the `walletdb` package.
