Require any Router also implements MessageRouter

Require any Router also implements MessageRouter

ChannelManager is parameterized by a Router in order to find routes when
sending and retrying payments. For the offers flow, it needs to be able
to construct blinded paths (e.g., in the offer and in reply paths).
Instead of adding yet another parameter to ChannelManager, require that
any Router also implements MessageRouter. Implement this for
DefaultRouter by delegating to a DefaultMessageRouter.