Fix CoI to APC mapping

Fix CoI to APC mapping

  * Route forwarded request based on mapped APC, not the original COI
  * Refactor COI/APC mapping code out of tr_tids_req_handler(), which
    remains in desperate need of refactoring for clarity
  * Use accessors instead of direct reference to structure elements in a
    few places (still more to convert)
  * Don't assume TR_NAME buf is null-terminated (it always is AFAIK, but
    is not required by the data structure). Still more of these to fix
  * Rename tid_req_set_rp_orig_coi() to _set_orig_coi(). It's not exported
    as part of the public API and was not used in our code. I think this
    was originally a copy/paste error.

This resolves https://bugs.launchpad.net/moonshot-tr/+bug/1765681