Performance regression in controller-rest-cars-perf

The regression may be in clustering code, or in Restconf code.
This affects both Boron and Carbon snapshots, in comparison with Beryllium snapshots.

The biggest difference is in Add_Cars test case, which uses a script [0] to send single POST request to create 10000 car items. But also Add_People test case takes longer, compare Boron durations [1] to Beryllium [2].

[0] https://github.com/opendaylight/integration-test/blob/master/tools/odl-mdsal-clustering-tests/scripts/cluster_rest_script.py
[1] https://jenkins.opendaylight.org/releng/view/controller/job/controller-csit-1node-rest-cars-perf-only-boron/870/robot/controller-rest-cars-perf.txt/
[2] https://jenkins.opendaylight.org/releng/view/controller/job/controller-csit-1node-rest-cars-perf-only-beryllium/116/robot/controller-rest-cars-perf.txt/