Enable -Wextra C++ warnings to Clang options

Investigate adding additional warnings to the set we currently have - specifically -Wextra in Clang which in theory is a well considered set of options which Clang developers recommend - see https://softwareengineering.stackexchange.com/questions/122608/clang-warning-flags-for-objective-c-development/124574#124574 for some context / background.

Try adding -Wextra to tlm/cmake/Modules/CouchbaseClangxxOptions.cmake, and investigate if the signal-to-noise ratio seems good enough for us to enable by default.

We may have to prune some of then if they are too noisy (a quick test locally suggests -Wunused-parameter may fall into this category).