Purpose of this pull request

This PR fix the behavior of thread context classloader be set to null when job done or submit job done. If other thread is using this classloader for load class, it may not be able to find an available classloader (because it thread context classloader be set to null). So we should ensure that the number of references to the classloader is 0 before trying to clean up the classloader in the thread.

Does this PR introduce any user-facing change?

no

How was this patch tested?

add new test.

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If you are contributing the connector code, please check that the following files are updated:
  1. Update change log that in connector document. For more details you can refer to connector-v2
  2. Update plugin-mapping.properties and add new connector information in it
  3. Update the pom file of seatunnel-dist
• Update the release-note.