Use parallel build to speed up building and running tests

Use parallel build to speed up building and running tests

Enable parallel building and parallel test-execution
Provide a configuration to still build/test serially on CI
via CI_BUILD=TRUE gradle --no-parallel --max-workers=1 clean build
Adjust Jenkins DSL to set CI_BUILD to avoid problems in CI
Adjust TestAllFiles to not fail when parallel builds are enabled
Reduce memory settings to reduce requirements on build-environments
Add gradle plugin to list task-dependencies

Thanks to Andreas Reichel for the PR

Closes #275

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1894812 13f79535-47bb-0310-9956-ffa450edef68