[release] LoRa Server 0.23.1

[brocaar]

This release fixes a couple of bugs

Changelog

Features:

• LoRa Server sets a random token for each downlink transmission.

Bugfixes:

• Add missing nil pointer check for Time
  (#280)
• Fix increase of NbTrans (re-transmissions) in case of early packetloss.
• Fix decreasing NbTrans (this only happened in case of data-rate or TX
  power change).