Consider refactoring plugin to use "virtual" CPS VM threads

This plugin came up in discussion on Jenkins JIRA:

https://issues.jenkins.io/browse/JENKINS-44085

Jesse Glick made this comment:

https://issues.jenkins.io/browse/JENKINS-44085?focusedCommentId=405935&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-405935

> From a brief glance at https://github.com/jenkinsci/concurrent-step-plugin I would say that it is designed incorrectly (confuses “native” Java threads with “virtual” CPS VM threads) and should not be used. Most or all of its steps probably could be reimplemented correctly while using the same Pipeline script interface.