Following the changes made in ff99bb07, Tomcat’s WebAppClassLoader is
set as the thread context class loader of the main thread. If Tomcat
was stopped while the main thread was still running (typically as a 
result of a startup failure), this had the unwanted side-effect of
causing Tomcat to report that the application had started a thread named
main and had failed to stop it.

This commit updates TomcatEmbeddedServletContainer so that, during stop
processing, the current thread’s context class loader is reset before
Tomcat is stopped. This prevents Tomcat from incorrectly believe that
the application has started and failed to stop the main thread.

Closes gh-5357