lr-wpan: Pending transaction list and association support params.

Part of the ongoing changes described in here

This MR adds managing functions to work with the "pending transaction list". The "pending transaction list" is necessary for the inclusion of Association to the lr-wpan module. While the pending transaction list exists in some form in the current implementation, it does not include any managing functions (enque, deque, purge). This MR adds these functions.

Additionally, structures and callbacks related to Association are added in this MR for future additions that will require these.