crimson/osd: add more privacy

crimson/osd: add more privacy

* reorder the member variable and member function definitions so the
  private members are listed at the end of class. as requested by
  the CodingStyle
* mark some member variables and member functions private, as long as
  they are not used by other classes or non-friend classes.
* document UrgentRecovery
* refactor BackgroundRecovery::get_scheduler_params() to dedup
  the comments

Signed-off-by: Kefu Chai <kchai@redhat.com>