Code modernisation opportunities (c++17)

[oschaaf]

In https://github.com/envoyproxy/nighthawk/pull/412 two clang-tidy checks have been disabled. It's possible that Envoy may revert back to c++14, and until we are sure that won't happen, we should probably not rely on c++17isms.

This issue tracks following up on enabling of the following checks: -modernize-use-nodiscard -modernize-concat-nested-namespaces