When an app is deployed to Tomcat, all of the application's startup
is performed with a WebAppClassLoader being the thread context
class loader. When an app is using embedded Tomcat, the
WebAppClassLoader is created as part of the application starting but
is never set as the thread context class loader. This difference
in TCCL can cause problems. For example, it breaks the use of JNDI
during application startup with embedded Tomcat.

This commit updates the embedded Tomcat servlet container to set
the TCCL to be the WebAppClassLoader once the Tomcat context has
been started. Once Tomcat is stopped, it sets the TCCL back to the
ClassLoader that loaded it.

Closes gh-2308