[Java] Fix the problem that flight perf test hangs endlessly

[asfimport]

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.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

PRs and other links:

• GitHub Pull Request #5799

_{Note: This issue was originally created as ARROW-7106. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 5799
#5799