(bug 43595) Add a cache version variable to SiteArray

Added a $cacheVersion variable to SiteArray to prevent problems
on SiteArray format changes due to a Memcached value of the object
being stored in the outdated format. This already caused disruption
on wikidata.org twice.

(This new version isn't tested as I don't have a memcached environment
with me atm.)

Change-Id: Ib290b465276b6be8b8753c27ea3f57d4597ec3c1