lightnvm: rename ssd_for_* macros to nvm_for_*

We're no longer using the openssd name for the work.
lightnvm: cleanup unused waiting_ws data structure and comments
lightnvm: remove buffered reads until a later stage
lightnvm: more cleanups to make it lkml ready
lightnvm: further cleanups

 * Comments
 * Code style
 * Move page_is_fast to hints.c