common/pick_address: refactor pick_addresses()

common/pick_address: refactor pick_addresses()

* consolidate the logic handling CEPH_PICK_ADDRESS_PREFER_IPV4 using
  std::sort(). this might be overkill. but it helps to explain
  what CEPH_PICK_ADDRESS_PREFER_IPV4 is for, and helps to dedup
  the code to order the addresses.
* let fill_in_one_address() return an optional<entity_addrvec_t>.
  more readable this way
* early return if the required address is not found, instead of
  checking variables like ipv4_r
* rename fill_in_one_address() to get_one_address() to reflect
  the change of the function's return value's type

Signed-off-by: Kefu Chai <tchaikov@gmail.com>