IRRd 4.1.0 beta 4

https://irrd.readthedocs.io/en/v4.1.0b4/releases/4.1.0.html

• Fix #271 - Add synthethic NRTM. (#325)
• Ref #323 - Ensure correct error logging in preload updates
• Enable SO_REUSEADDR on TCP sockets.
• Fix #320 - Fix Python 2.6 compatibility (#327)