OnuDown indication can collide with FlowDeletion resulting in wrong flowDelete error indication

When flowDeletion is received just before or shortly after OnuDown indication by the OnuAdapter then both processes are processed in parallel with the result that some internal-error is given as response on the FlowDelete RPC for the rwCore. This behavior is not intended and should be avoided.
This in turn might have further implication after the ONU comes up again. It is a possible reason for the rwCore not supplying the flow configuration (and techProfile configuration as well) anymore.
Observed in Multi-Olt regression within the soft-reboot test (1t8Gem) here: https://jenkins.opencord.org/view/All%20Jobs/job/periodic-voltha-multiple-olts-openonu-go-test-bbsim/579/