Today I monitored threads created within our application while running on tomcat 8.5.3x64 because of an issue in my code.
While doing that I noticed that there were two threads called Thread-7 and Thread-8 (name was stable during a lot of starts but only on the first start) that I wasn't able to find in my code so I debugged a bit deeper and found that they were created in org.apache.isis.core.runtime.threadpool.ThreadPoolSupport:76 during start-up.
After that they are going to state PARK and reside even when undeploying the application from tomcat.
I haven't looked much at your code but I believe you should call shutdown on the ThreadPoolExecutor that you are using once you are finished or in PreDestroy situation. At least I had a very similar issue in my application and shutdown() solved it.
Please find attached a screenshot of IntelliJ view during debug, the screenshot of VisualVM and the catalina log from tomcat complaining about these two threads.