v3.0.0

• Merged all different flavours of packet forwarder into one unique lora_pkt_fwd
  Note: Various flavours can still be achieved using the corresponding
  global_conf.json.XXX file provided in lora_pkt_fwd/cfg.
• Added downlink "just-in-time" scheduling to optimize downlink capacity.
• Updated Gateway <-> NetworkServer protocol to describe the new format of
  "tx_ack" message.
• Added "Listen-Before-Talk" configuration.
• Splitted reset_pkt_fwd.sh script in 2 different scripts:
  • reset_lgw.sh, provided with the HAL (lora_gateway)
  • update_gwid.sh, provided with lora_pkt_fwd