The former behaviour to always use a SHA-256 hash of the server
certificate for the tls-server-end-point channel binding was
incorrect with regard to RFC5929, which specifies if the signature
algorithm uses a single hash function, that hash function should be
used, unless the signature algorithm uses MD5 or SHA-1, in which
case the channel binding should use an SHA-256 hash.
This change attempts to align with the behaviour of OpenSSL, used
by PostgreSQL, in using the signature algorithm's hash function.
