DispatcherPortlet never uses a resource forward on Liferay

Issue: SPR-10791
"acceptExisting" flag allows for sharing per cacheManagerName on EhCache 2.5+

As a benefit over the "shared" flag, this allows for sharing a specific CacheManager in a system with potentially multiple CacheManagers involved, and it supports controlled shutdown by the EhCacheManagerFactoryBean that actually created the CacheManager.

Issue: SPR-11178