wifi: Misc fixes and code cleanup

This merge request is mainly to:

• fix issues #27 (closed) and #33 (closed)
• try harder to preserve the lifetime of MSDUs
• further cleanup some portions of code

Version 2

• Add commits to improve handling of the originator's starting sequence number and prevent QosTxops from sending old MPDUs to MacLow (fix issue #45 (closed)).
• The retransmission queue of the BlockAckManager is converted to a WifiMacQueue. This allows to remove some methods of BlockAckManager and also to get rid of the MacLow aggregation queue.

Version 3

• Rebased on current master

Version 4

• Addressed comments by Sebastien, Tom and Rediet

Version 5

• Extended tests based on comments by Sebastien