etcd+channeldb: fix linter issues, rename receiver

With this commit we address some issues the linter picked up after
touching older code.

kvdb/etcd/fixture.go

@@ -11,7 +11,7 @@ import (
 
 	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/stretchr/testify/require"
-	"go.etcd.io/etcd/client/v3"
+	clientv3 "go.etcd.io/etcd/client/v3"
 	"go.etcd.io/etcd/client/v3/namespace"
 )