this commit adds a complete DL HARQ entity to the MAC of the UE.
It also refactors demux into an own class and adapts the PHY-MAC
interface to use the new MAC capabilities.
this patch adds a basic UL HARQ entity for NR.
The patch also updates some interfaces between MAC/RA/HARQ,
i.e. get_temp_crnti().
It also adds a Msg3 unit test.
* refactor some common methods to mac_common.cc
* add common mux_base class
* move UL PDU generation to mux class
* add logical channel registration to MAC, MUX, BSR
* add initial proc BSR
* add basic MAC NR test
* rework MAC interfaces