types/netmap, ipn/ipnlocal, control/controlclient: rename NodeMutationAdd to NodeMutationUpsert

NodeMutationAdd was a misleading name: a PeersChanged entry in a
MapResponse can represent either a truly new peer or a full
replacement for an existing peer that couldn't be expressed as a
PeerChangedPatch. Calling it "Add" implied it was always a completely
new node, which is wrong.  (I'd changed my mind on the design of
mapping add/delete events to NodeMutations halfway through #19607 and
forgot to update the name, even though I'd updated half the docs)

Rename it to NodeMutationUpsert to reflect the actual semantics: the
node should be inserted or replaced in the peer map regardless of
whether it already existed.

Updates #19607
Updates #12542

Change-Id: Iebd3daddb3318cba02e115a1b184fcb3ee8f83d6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/node_backend.go

@@ -675,7 +675,7 @@ func (nb *nodeBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 
 	for _, m := range muts {
 		switch m := m.(type) {
-		case netmap.NodeMutationAdd:
+		case netmap.NodeMutationUpsert:
 			nid := m.Node.ID()
 			mak.Set(&nb.peers, nid, m.Node)
 			for _, ipp := range m.Node.Addresses().All() {