Code modernisation opportunities (c++17)

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