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>
2c965ab540