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ARROW-7506: [Java] JMH benchmarks should be called from main methods

Some benchmarks are called as unit tests in our current code base. They should be called from main methods, because:

1. This is the recommended way of writing JMH benchmarks. The automatically generated benchmarks are called from main, and sample benchmarks provided by JMH [1] are also called from main.

2. Some compiler does not support calling JMH as unit test. For example, the "javac with error prone" reports the following error:

Error:(100, 15) java: [JUnit4TearDownNotRun] tearDown() method will not be run; please add JUnit's @After annotation
(see https://errorprone.info/bugpattern/JUnit4TearDownNotRun)
Did you mean '@After'?

3. When run as a unit test, enable assert flag will be turned on by default, so some test/debug operations will be performed. This will distort the benchmark result data. For example, a related discussion can be found in [2].

[1] https://hg.openjdk.java.net/code-tools/jmh/file/tip/jmh-samples/src/main/java/org/openjdk/jmh/samples/
[2] https://github.com/apache/arrow/pull/5842#issuecomment-558082914

Closes #6132 from liyafan82/fly_0107_bench and squashes the following commits:

0ebc7cc65 <liyafan82>  JMH benchmarks should be called from main methods

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>