imove fr_pair_list_t to use tlists

imove fr_pair_list_t to use tlists

and do various other cleanups and fixes as necessitated by that.

* doing actual work inside of the dcursor insert/remove callbacks
* adding fr_pair_list_t* to fr_pair_verify()
* adding PAIR_VERIFY_WITH_LIST() to verify that a VP is in a list
* ensure that pair_init_from_da() is always used, instead of
  manually doing things in multiple places