As usual, we keep with the last two Go versions, which means the last
year of Go release (they release every 6 months or so).
This change means we can now use the type params (generics) in the
project!
In this commit, we modify the `macaroons.NewService` consturctor to
accept the main interface rather than the raw DB. This allows us to use
other backends other than bolt or the kvdb interface to store the
macaroon root keys.
We also create a new ExtendedRootKeyStore interface that implements some
of the more advanced features we use such as macaroon encryption and
password rotation.
