prefer prefix ++/--operator for e.g. iterators for performance reasons

A new bunch of performance related changes from postfix to prefix ++/--operator for e.g. iterators.

Includes two fixes for:
- make sure range_iter isn't out of range in clone_objs_impl() (rgw/rgw_rados.cc)
- use empty() instead of size() == 0

v2 of pull request #110