In this commit, we add a new implementation of the FeeEstimator
interface: the BtcdFeeEstimator. This implementation of the
FeeEstimator is backed by an active bcd instance. Any requests to query
for the current fee for a given confirmation target are proxied to this
active bcd instance.
In this commit, we extend the FeeEstimator methods to allow them to
return an error. This is required as most implementations aside from
the static fee estimator will want to be able to return errors to users
to indicate the inability to properly estimate fees.
In this commit, we move the FeeEstimator interface into a distinct file
as follow up commits will begin to flesh out the interface with
additional implementations.
