By default, allow SCT timestamps to be up to 5 minutes in the future

As requested in
https://github.com/openssl/openssl/pull/1554#issuecomment-246371575

.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1554)