Fixes #3787 Message service streaming sink was created using direct call to rpc::client::make_sink. This in turn needs a new socker, which it creates completely ignoring what underlying transport is active for the client in question. Fix by retaining the tls credential pointer in the client wrapper, and using this in a sink method to determine whether to create a new tls socker, or just go ahead with a plain one. Message-Id: <20181010003249.30526-1-calle@scylladb.com>