Depends on btcsuite/btcwallet#757.
Pulls in the updated version of btcwallet and walletdb that have the DB
interface enhanced by their own View() and Update() methods with the
reset callback/closure supported out of the box. That way the global
package-level View() and Update() functions now become pure redirects.
This commit adds gives BtcdNotifier access to the block cache and wraps
its GetBlock method so that the block cache's mutex map for the specific
hash is used.
