rgw: clean-up use enum class for stats category

rgw: clean-up use enum class for stats category

The stats entries for rgw buckets has a category, which used a
combination of uint8_t and enum RGWObjClass. Clean this up by
converting RGWObjClass to an enum class and using that
throughout. This provides type safety and better code clarity. Also,
add some source code documentation.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>