This was preventing tailscaled from shutting down properly if there were
active sessions in certain states (e.g. waiting in check mode).
Signed-off-by: Maisem Ali <maisem@tailscale.com>
And add a private context type in the process.
Updates #3802
Change-Id: I257187f4cfb0f2248d95b81c1dfe0911ef203b60
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>