control/controlhttp/controlhttpserver: detach websocket conns from request context

The websocket upgrade path bound the returned conn's reads and writes
to the context passed to AcceptHTTP via wsconn.NetConn. That context is
typically an http.Request context, which net/http cancels once the
calling handler returns, so a caller that served the accepted conn
beyond its handler's lifetime had the conn killed out from under it.
The hijack-based HTTP/1 upgrade path has no such binding.

Make the two paths consistent: ctx now only bounds the handshake (its
deadline, if any, is applied to the conn by controlbase.Server) and the
returned conn's lifetime is the caller's responsibility. Document that
contract on AcceptHTTP and add a regression test that uses a
websocket-accepted conn after the accepting handler has returned.

Updates tailscale/corp#46806
Updates tailscale/corp#29053

Change-Id: I4fc4ea5cddc2c6174fdf21f8d832f2e0984a7533
Co-authored-by: Adriano Sela Aviles <adriano@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>