Fills a Route TLV. When @p aNeighbor is not `nullptr`, we limit the number of router entries to `kMaxRoutersInRouteTlvForLinkAccept` when populating `aRouteTlv`, so that the TLV can be appended in a Link Accept message. In this case, we ensure to include router entries associated with @p aNeighbor, leader, and this device itself.
Handles a deferred ack. Some radio links can use deferred ack logic, where a tx request always report `HandleSentFrame()` quickly. The link layer would wait for the ack and report it at a later time using this method. The link layer is expected to call `HandleDeferredAck()` (with success or failure status) for every tx request on the radio link.