openonuAdapterGo - memory leak seen in long term tests

We noticed that the memory in the openonu adapter keeps going up.

• basic state 64 ONUs with flows
• disable PON ports
• delete ONU flows
• delete ONUs
• enable PON ports
• discover ONUs
• push ONU flows
  In the initial state (64 ONUs with flows) the openonu-adapter is at less than 100MB, after 29x runs the memory is at 1.2GB.

Issue should be visible in master and other versions too, but current observation was made in voltha 2.8:
• voltha-go 2.9.5
• adtranolt-adapter 1.9.0 (auf Basis openolt-adapter 3.5.4)
• openonu-adapter-go 1.3.15
• ofagent-go 1.6.5
• voltha-onos 5.0.2