Fix last clang-tidy warnings

Per @springmeyer and @joto 's thoughts:

We have a few clang-tidy warnings needing fixed.

They should be failing the build, but are not due to https://github.com/mapbox/node-cpp-skel/issues/105.

These are what show up for me with make tidy locally on OS X:

../src/filters.cpp:50:13: error: initializing non-owner 'Filters *const' with a newly created 'gsl::owner<>' [cppcoreguidelines-owning-memory,-warnings-as-errors]
            auto* const self = new Filters();
            ^
../src/shave.cpp:152:9: error: initializing non-owner 'AsyncBaton *' with a newly created 'gsl::owner<>' [cppcoreguidelines-owning-memory,-warnings-as-errors]
        auto* baton = new AsyncBaton();
        ^
../src/shave.cpp:342:60: error: deleting a pointer through a type that is not marked 'gsl::owner<>'; consider using a smart pointer instead [cppcoreguidelines-owning-memory,-warnings-as-errors]
                                                           delete reinterpret_cast<std::string*>(hint);
                                                           ^
../src/shave.cpp:341:66: note: variable declared here
                                                       [](char*, void* hint) {
                                                                 ^
../src/shave.cpp:353:5: error: deleting a pointer through a type that is not marked 'gsl::owner<>'; consider using a smart pointer instead [cppcoreguidelines-owning-memory,-warnings-as-errors]
    delete baton;
    ^
../src/shave.cpp:329:5: note: variable declared here
    auto* baton = static_cast<AsyncBaton*>(req->data);
    ^

Now, I noticed that @joto just disabled this clang-tidy warning (`cppcoreguidelines-owning-memory) today in vtzero which indicates to be we may want to consider the same thing: https://github.com/mapbox/vtzero/commit/6822c59e17a6d51196d78fce6d2a5d383522c6e9

I think the cppcoreguidelines-owning-memory only makes sense if you buy into the whole gsl library. Each project has to decide whether it makes sense for them, but I think the gsl library makes more sense for higher level applications while lower-level libraries a) need more bit-twiddling and pointer-twiddling etc. anyway so they need to disable lots of these warnings and b) any extra dependency on low-level libraries makes they use that much harder.

Run make tidy on OS X and inspect the reported diagnostics in src/filters.cpp and src/shave.cpp. Review the existing clang-tidy configuration and the linked vtzero change before deciding whether to address the ownership warnings or disable that check. Done means these diagnostics no longer appear and the intended tidy behavior is reflected in the build configuration.