## What is the purpose of the change This PR bumps the checkstyle version to 8.9, which is a prerequisite for compiling with java 9.
## Brief change log * bump version to 8.9 * updated IDE setup docs * fixed a number of checkstyle violations that weren't detected previously ## Verifying this change I had to setup a local VM with jdk 9 to encounter the issue and verify the fix. Travis is unfortunately unreliable for this, as some jdk 9 incompatibilities that I encountered locally didn't occur on travis (which is actually **very** problematic). [ Full content available at: https://github.com/apache/flink/pull/6618 ] This message was relayed via gitbox.apache.org for [email protected]
