Modernize the Core/Network Benchmarks testrunner

Since roughly 08.08.2021, we no longer collect data (run?) the core&network benchmarks. See https://testresults.qt.io/grafana/d/000000029/core-and-network-benchmarks?orgId=1&var-case_name=EventsBench&var-test_name=postEvent&var-data_row=All&var-branch=All&var-host=All&from=now-1y%2Fy&to=now-1y%2Fy

The testrunner script relies on COIN storagestructs, and has some issues with it's async libraries littering file handles. The script needs to be modernized and updated to avoid deserialization of COIN storagestruct objects, as well as rethinking how async jobs are handled to avoid leaking memory and file handles.