vinyl: followup malloc range->begin/end and page->min_key

 - Fix memory leak in vy_task_compact_new() - split_key is never freed.
   We don't need to copy it there, because page->min_key is guaranteed
   to stay alive during vy_task_split_new().
 - Do not call vy_key_dup() on record's begin/end on recovery. Simply
   pass them directly to vy_range_new() - the latter will copy them
   for us.
 - Mark begin/end arguments of vy_range_new() const.