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ARROW-7106: [Java] Fix the problem that flight perf test hangs endlessly

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.

Closes #5799 from liyafan82/fly_1108_hang and squashes the following commits:

29de5984f <liyafan82>  Fix the problem that flight perf test hangs endlessly

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>