This adds a lighter mechanism for endpoint updates from control.
Change-Id: If169c26becb76d683e9877dc48cfb35f90cc5f24
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
So the NetworkMap-from-incremental-MapResponses can be tested easily.
And because direct.go was getting too big.
No change in behavior at this point. Just movement.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>