changing back the rxtx thread and add another worker thread for coding

openair1/PHY/LTE_TRANSPORT/dlsch_coding.c

openair1/PHY/defs.h

openair1/SCHED/phy_procedures_lte_eNb.c

openair2/LAYER2/MAC/eNB_scheduler_phytest.c

targets/RT/USER/eNB_usrp.gtkw

targets/RT/USER/lte-enb.c