Here's legal jira: https://issues.apache.org/jira/browse/LEGAL-399
TL;DR: JMH is GPL + "Classpath" exception, and that exception allows to
link JMH with independent modules to produce an executable regardless of
the license terms of these independent modules.
In other words, no harm is made even if we release `ubenchmark` to Central
JMH use would be valid even if Calcite actually required JMH to function.
It does not mean I want to use ubenchmark code in something like SqlToRel,
but it is for clarification purposes.