Clear the GroovyClassLoader cache before compiling

Since we don't use the cache, it's okay to clear it entirely if needed,
Elasticsearch maintains its own cache for compiled scripts.

Adds loader.clearCache() into a listener, the listener is called when a
script is removed from the Guava cache.

This also lowers the amount of cached scripts to 100, since 500 is
around the limit some users have run into before hitting an out of
memory error in permgem.

Fixes #7658