This commit introduces a separate thread (l1_tx_thread) that processes
gNB DL/TX slots. It receives a message from the ru_thread when a new
slot started, and starts processing.

The DL part of the scheduler is run in the l1_tx_thread.

The reorder thread is removed, as reordering with this scheme is not
necessary anymore.

The main advantage of this version is that the TX can be scheduled
earlier (sl_ahead). Further, there can no longer be race conditions in
the TX path, since the scheduler/L2, TX L1 processing and the RU tx
function are all execute in the same thread (L1_tX_thread).
Co-authored-by: kiran <saikiran@iitj.ac.in>