update to match new ExtractPublicKey API

This is a "more correct" API and we might as well fix this before it becomes
used all over the place. The new API returns ErrNoPublicKey when there is no
inlined key.

This *also* fixes a bug where the datastore-backed keystore would panic if
`ExtractPublicKey` returned `nil, nil`.

pstoremem/keybook.go

@@ -49,7 +49,7 @@ func (mkb *memoryKeyBook) PubKey(p peer.ID) ic.PubKey {
 		return pk
 	}
 	pk, err := p.ExtractPublicKey()
-	if err == nil && pk != nil {
+	if err == nil {
 		mkb.Lock()
 		mkb.pks[p] = pk
 		mkb.Unlock()