ONU goes from ACTIVE to DISCOVERED state after authentication

When an ONU's operational state is ACTIVE with `tech-profile-config-download-success` state in VOLTHA, authenticating this ONU makes the ONU go back into DISCOVERED state and `omci-flows-addition-failed-retrying` or`omci-flows-deletion-failed-retrying` states in VOLTHA.  ONU doesn't get recovered from this state.

Issue is reproducible.

Note: This issue is observed while testing the following voltha patches on voltha-1.7 branch

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https://gerrit.opencord.org/#/c/13854/
https://gerrit.opencord.org/#/c/13818/
https://gerrit.opencord.org/#/c/13788/
https://gerrit.opencord.org/#/c/14028/

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Steps to reproduce the issue:

• Add OLT/ONU with a corresponding whitelist/Subscriber through NEM
• When the ONU comes into ACTIVE/tech-profile-config-download-success state
• Perform authentication
• Authentication succeeds 
• Immediately noticed that the ONU went into DISCOVERED sate
• ATT-Service instance corresponding to the above ONU goes back into `AWAITING` state instead of `APPROVED` state

Logs are attached