Add some accessors.

Author: Remi Gacogne <rgacogne-github@coredump.fr>
GH334: Add an OCSP_SINGLERESP_get0_id() accessor to the OCSP_CERTID of
a OCSP_SINGLERESP. It is possible to do it the other way around using
OCSP_resp_find(), but this is more efficient when you have a tree indexed
by OCSP_CERTID, like haproxy does. (This is also RT4251)

Author: Marek Klein <kleinmrk@gmail.com>
GH556: OCSP_resp_get_produced_at() accessor to the producedAt of a
OCSP_BASICRESP
GH555: TS_STATUS_INFO_get_status(), TS_STATUS_INFO_get_text() and
TS_STATUS_INFO_get_failure_info() accessors for a TS_STATUS_INFO

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>