tailcfg: introduce map version 8, for clients that support v6 node config.

For now, the server will only send v6 configuration to mapversion 8 clients
as part of an early-adopter program, while we verify that the functionality
is robust.

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlclient/direct.go

@@ -545,7 +545,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	}
 
 	request := tailcfg.MapRequest{
-		Version:    7,
+		Version:    8,
 		KeepAlive:  c.keepAlive,
 		NodeKey:    tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
 		DiscoKey:   c.discoPubKey,

tailcfg/tailcfg.go

@@ -478,6 +478,7 @@ type MapRequest struct {
 	//     5: 2020-10-19, implies IncludeIPv6, delta Peers/UserProfiles, supports MagicDNS
 	//     6: 2020-12-07: means MapResponse.PacketFilter nil means unchanged
 	//     7: 2020-12-15: FilterRule.SrcIPs accepts CIDRs+ranges, doesn't warn about 0.0.0.0/::
+	//     8: 2020-12-19: client can receive IPv6 addresses and routes if beta enabled server-side
 	Version     int
 	Compress    string // "zstd" or "" (no compression)
 	KeepAlive   bool   // whether server should send keep-alives back to us