go/zodb/zeo: Accept both int|bool for pickled message flags

It is already documented in pktDecodeZ comment that flags is int|bool.
However until now we were decoding it only as int.

As is it was working, but it will fail when receiving e.g.
invalidateTransaction message, because ZEO/py server actually uses bool
when sending it:

    https://github.com/zopefoundation/ZEO/blob/5.2.1-20-gcb26281d/src/ZEO/asyncio/base.py#L139

Fix it.
This will be covered by watch tests, when watch support will be added in a followup patch.

go/zodb/storage/zeo/proto.go

@@ -112,7 +112,14 @@ func pktDecodeZ(pkb *pktBuf) (msg, error) {
 
 	flags, ok := pickletools.Xint64(tpkt[1])
 	if !ok {
-		return m, derrf("flags: got %T; expected int", tpkt[1])
+		bflags, ok := tpkt[1].(bool)
+		if !ok {
+			return m, derrf("flags: got %T; expected int|bool", tpkt[1])
+		}
+
+		if bflags {
+			flags = 1
+		} // else: flags is already = 0
 	}
 	// XXX check flags are in range?
 	m.flags = msgFlags(flags)