Performance regression in cluster registration

The problem seems to be in all branches:

https://jenkins.opendaylight.org/releng/job/controller-csit-3node-clustering-ask-all-neon/247/robot/controller-clustering-ask.txt/Chasing%20The%20Leader/Unregister_Candidates_And_Validate_Criteria/

https://jenkins.opendaylight.org/releng/job/controller-csit-3node-clustering-tell-all-fluorine/220/robot/controller-clustering-tell.txt/Chasing%20The%20Leader/Unregister_Candidates_And_Validate_Criteria/

In Fluorine it started at around May 23rd so there multiple suspect patches:

https://git.opendaylight.org/gerrit/#/q/branch:stable/fluorine+project:controller

The test itself does:

1) Start a singleton registration flap on every controller instance with this RPC: /restconf/operations/odl-mdsal-lowlevel-control:register-flapping-singleton

2) Maintain the flap for 60 secs.

3) Stop the flap on every controller instance: /restconf/operations/odl-mdsal-lowlevel-control:unregister-flapping-singleton

4) Get flap count from above RPC response: <output xmlns="tag:opendaylight.org,2017:controller:yang:lowlevel:control"><flap-count>83</flap-count></output>

5) Add all the flaps for the 3 controller instances and divide the total by 60 secs.

Before the regression, the controller handled ~50 flaps/sec, after the regression is less than 5 flaps/sec.