Flight performance test (org.apache.arrow.flight.perf.TestPerf) is an important tool for tracking the current throughput of IPC. In this issue, we improve it in two ways:
1. We fix the problem that the test hangs endlessly after all runs have been finished. This is because the thread pool is not released.
2. We add a summary to the output report, so that we can easily evaluate the overall results for all runs.