MINOR: backend: extract conn hash calculation from connect_server()

MINOR: backend: extract conn hash calculation from connect_server()

On connection reuse, a hash is first calculated. It is generated from
various connection parameters, to retrieve a matching connection.

Extract hash calculation from connect_server() into a new dedicated
function be_calculate_conn_hash(). The objective is to be able to
perform connection reuse for checks, without connect_server() invokation
which relies on a stream instance.