* Remove unnecessary mutexes (there's no concurrency)
* Simplify LocalBackend.UpdateStatus using the StatusBuilder.WantPeers
field that was added in 0f604923d345ff, removing passing around some
method values into func args. And then merge two methods.
More remains, but this is a start.
Updates #9433
Change-Id: Iaf2d7ec6e4e590799f00bae185465a4fd089b822
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>