openoltAdapter: ONUs are undiscoverable after multiple OLT reboots

Our ADTRAN colleagues have found the following issue in the VOLTHA Adtran OLT Adapter that is also relevant for the openolt Adapter:

After multiple reboots (mostly 3) of the OLT it can happen that in OLT Adapter it is tried to stop not running routines (collector and heatbeatCheck). The attempt to stop this routine, however, returns only when the routine is started (which immediately terminates again). This messes up the whole OLT adapter handling and the ONUs can not be discovered anymore.

Steps to reproduce:
voltctl device create
voltctl device enable
--> OLTs and ONUs reachable
voltctl device reboot
--> OLTs and ONUs reachable
voltctl device reboot
--> OLTs and ONUs reachable
voltctl device reboot
--> OLTs reachable and ONUs missing