bucket: allow to allocate key on stack in Put()

As per `go build -gcflags -m ./... 2>&1`:

Old behaviour:
```
./bucket.go:148:31: leaking param: key
./bucket.go:192:42: leaking param: key
./bucket.go:271:22: leaking param: key
```

Now:
```
./bucket.go:148:31: key does not escape
./bucket.go:192:42: key does not escape
./bucket.go:271:22: key does not escape
```

Signed-off-by: Evgenii Stratonikov <fyfyrchik@runbox.com>

bucket.go

@@ -175,15 +175,17 @@ func (b *Bucket) CreateBucket(key []byte) (*Bucket, error) {
 	var value = bucket.write()
 
 	// Insert into node.
-	key = cloneBytes(key)
-	c.node().put(key, key, value, 0, common.BucketLeafFlag)
+	// Using new variable here is important: otherwise `key` param
+	// is marked as leaking and cannot be allocated on stack.
+	newKey := cloneBytes(key)
+	c.node().put(newKey, newKey, value, 0, common.BucketLeafFlag)
 
 	// Since subbuckets are not allowed on inline buckets, we need to
 	// dereference the inline page, if it exists. This will cause the bucket
 	// to be treated as a regular, non-inline bucket for the rest of the tx.
 	b.page = nil
 
-	return b.Bucket(key), nil
+	return b.Bucket(newKey), nil
 }
 
 // CreateBucketIfNotExists creates a new bucket if it doesn't already exist and returns a reference to it.
@@ -291,8 +293,10 @@ func (b *Bucket) Put(key []byte, value []byte) error {
 	}
 
 	// Insert into node.
-	key = cloneBytes(key)
-	c.node().put(key, key, value, 0, 0)
+	// Using new variable here is important: otherwise `key` param
+	// is marked as leaking and cannot be allocated on stack.
+	newKey := cloneBytes(key)
+	c.node().put(newKey, newKey, value, 0, 0)
 
 	return nil
 }