updates tailscale/tailscale#1909
This adds a new new NotifyWatchOpt that allows you to
receive PeerChange events (derived from node mutations)
on the IPN bus in lieu of a complete netmap. We'll continue
to send the full netmap for any map response that includes it,
but for mutations, sending PeerChange events gives the client
the option to manage it's own models more selectively and cuts
way down on json serialization overhead.
On chatty tailnets, this can vastly reduce the amount of
chatter on the bus.
This change should be backwards compatible, it is
purely additive. Clients that subscribe to NotifyNetmap will
get the full netmap for every delta. New clients can
omit that andinstead opt into NotifyPeerChanges.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>
87a2289e7e