Stop opening an IPN bus subscription with NotifyInitialNetMap purely to
read the current netmap once. Use the LocalAPI debug current-netmap
action (added in 159cf8707) instead, which returns the current netmap
synchronously without subscribing to the bus.
Updates #12542
Change-Id: I8aa2096d65aaea4dfe62634f03ce06b5470e0e51
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>