To make ExitDNS cheaper.
Might not finish client-side support in December before 1.20, but at
least server support can start rolling out ahead of clients being
ready for it.
Tested with curl against peerapi.
Updates #1713
Change-Id: I676fed5fb1aef67e78c542a3bc93bddd04dd11fe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>