proto.NotPrimaryMaster: Fix .Primary data type

The '.Primary' attribute of the 'NotPrimaryMaster' packet has been
assigned to 'NodeID' data type. This is incorrect, because the data
doesn't represent the ID of the node, but an index of the
'.KnownMasterList' [1]. In the old protocol NEO/py therefore also
used 'PSignedNull' instead of 'PUUID' [2]. This patches fixes the data
type of '.Primary' and uses 'int8' instead of 'NodeID'. Technically this
doesn't make any difference, but semantically for human beings the code
is easier to understand now.

[1] https://lab.nexedi.com/nexedi/neoppod/blob/c6453626/neo/lib/handler.py#L161
[2] https://lab.nexedi.com/nexedi/neoppod/blob/c6453626/neo/lib/protocol.py#L716

go/neo/proto/proto.go

@@ -457,7 +457,7 @@ type AnswerPrimary struct {
 //
 //neo:nodes SM -> *
 type NotPrimaryMaster struct {
-	Primary         NodeID // XXX PSignedNull in py
+	Primary         int8
 	KnownMasterList []struct {
 		Address
 	}

go/neo/proto/zproto-marshal.go

@@ -807,7 +807,7 @@ func (p *NotPrimaryMaster) neoMsgDecodeN(data []byte) (int, error) {
 	if len(data) < 8 {
 		goto overflow
 	}
-	p.Primary = NodeID(int32(binary.BigEndian.Uint32(data[0 : 0+4])))
+	p.Primary = int8(int32(binary.BigEndian.Uint32(data[0 : 0+4])))
 	{
 		l := binary.BigEndian.Uint32(data[4 : 4+4])
 		data = data[8:]
@@ -882,7 +882,7 @@ func (p *NotPrimaryMaster) neoMsgDecodeM(data []byte) (int, error) {
 		if err != nil {
 			return 0, mdecodeErr("NotPrimaryMaster.Primary", err)
 		}
-		p.Primary = NodeID(v)
+		p.Primary = int8(v)
 		nread += uint64(len(data) - len(tail))
 		data = tail
 	}