Support offloading to cluster wide cache of hst page caches in ehcache

if memcached is available, offloading ehcache page responses has many advantages regarding caching options. Namely, ehcache we need to flush after any change below the content tree : We cannot keep a page for say another minute, because then different cluster nodes might start to give responses that are too different. However, if we keep one single location for the 'global' cache, we can easy cache a page for a minute, as the entire cluster accesses this same cache. Also, cluster nodes share then their cached responses, which make sure that less pages need to be rendered by the webapps.

Implementation

1) We keep the cached responses locally in (blocking) ehcache
2) We also offload cached responses to memcached with some timeToLive
3) Whenever a change in content tree is triggered, all cluster nodes flush the ehcache
4) When a request comes in for a page that is not in ehcache, first memcached is asked for the page : if present, this page is cached locally again and returned