Plumb throughout the codebase as a replacement for the mixed use of
tailcfg.MachineKey and wgkey.Private/Public.
Signed-off-by: David Anderson <danderson@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>
