wifi: Introduce the QosFrameExchangeManager class
This MR builds on top of !477 (merged) (the first new commit is "wifi: Add support for setting the Queue Size subfield in the QoS control field") and introduces the QosFrameExchangeManager class that adds the support for TXOPs. Transmission failures are handled according to the specs (PIFS or backoff recovery) and the concept of the TXOP holder is added.