debugging information in lte-enb, addition of external reference option in lte-softmodem and usrp_lib.cpp

changes for oaisim to function with new threading architecture and trx_read/trx_write emulation. Tested for 5/10 MHz TM1. Multiple-antenna mode fails.

usrp 20 MHz on 30.72 master clock. fixed memory leak in prach.c for 20 MHz with 3/4 sampling. added default attributes to threads in lte-enb.c