DIH: parameter "cacheKey" was inadvertently renamed "cachePk"

CachedSqlEntityProcessor supports an obscure alternate to the "where" parameter. Instead of <entity ... where='id=x.id' /> , users can use <entity ... cacheKey="id" cacheLookup="x.id" /> However, this was broken with SOLR-2382. "cacheKey" was accidently renamed "cachePk". For the sake of those who might be using this undocumented syntax and want to upgrade, I think it should be put back to "cacheKey". Also, I will add documentation in the wiki.