Correctly handle SIGINT/stop signal in nr-softmodem

• Correctly handle SIGINT, so that the nr-softmodem stops regularly
• Implement abortTpool() to stop the complete Tpool (the previous abortTpool() function has been renamed to abortTpoolJob())
• Implement abortNotifiedFIFO() to unblock an existing FIFO (the previous abortNotifiedFIFO() function has been renamed to abortNotifiedFIFOJob())
• Improve the Tpool documentation markup, see thread-pool.md
• Correctly stop the USRP: it is possible to start the nr/lte-softmodem immediately after it has been stopped, without the need to wait: should solve #519 (closed)
• Some memory fixes: on L1, for NFAPI PUCCH PDUs, and a big memory leak for UP traffic in UL at the GTP