This commit extends the kvdb interface in a backwards compatible way
such that we'll be able to prefetch all keys in a bucket in one go reducing the
number of roundtrips.
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.
We only want to register the bbolt DB backend ("bdb") when we're not
compiling for a JS/WASM build targets. That's why we want to have that
import in only one file that we can add a build tag to. We remove it in
two other places since only one import is enough anyway.
