LDEV-6240 skip findLoadedClass in EnvClassLoader, clear callerCache on RPC flush

[zspitzer]

Summary

Backport of #2775 to 6.2.

• Remove findLoadedClass() from EnvClassLoader.loadClass() — it returns classes from the JVM's internal cache which cannot be cleared after a PhysicalClassLoader flush
• Add clearCallerCache() to EnvClassLoader and call it from PhysicalClassLoader.flush() for RPC classloaders
• After flush, stale classes from the old classloader caused isAssignableFrom failures in ServiceLoader and other JDK code

https://luceeserver.atlassian.net/browse/LDEV-6240

Test plan

• test/tickets/LDEV6240.cfc — 5 tests all passing