BUG/MINOR: sample/ssl: Fix digest converter for openssl < 1.1.0

BUG/MINOR: sample/ssl: Fix digest converter for openssl < 1.1.0

The EVP_MD_CTX_create() and EVP_MD_CTX_destroy() functions were renamed to
EVP_MD_CTX_new() and EVP_MD_CTX_free() in OpenSSL 1.1.0, respectively. These
functions are used by the digest converter, introduced by the commit 8e36651ed
("MINOR: sample: Add digest and hmac converters"). So for prior versions of
openssl, macros are used to fallback on old functions.

This patch must only be backported if the commit 8e36651ed is backported too.