Package `pkg/maintainer/spv` should not depend on `btcd` directly

[lukasz-zimnoch]

The btcd package is a 3rd party dependency meant to be used only within pkg/bitcoin package which isolates it from the client code. However, pkg/maintainer/spv depends on btcd directly which makes it leak beyond pkg/bitcoin. We should fix that and ensure btcd is not used directly by any other package except pkg/bitcoin.