This has been a TODO for ages. Time to do it.
The goal is to move more network state accessors to netmon.Monitor
where they can be cheaper/cached.
Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299
Change-Id: I60fc6508cd2d8d079260bda371fc08b6318bcaf1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>