The runtime call reads the cert and key options before the SNICallback,
so a cert or key initializer after the callback that reassigned the
callback's binding changed the callback the server used, and the server
could serve the wrong certificate. When SNICallback comes before a cert
or key entry, the callback is now saved just before the next of them is
evaluated. With the callback read in order, a callback expression with
side effects lowers instead of being refused.