partially clearing the keys fix: #14

[tvatavuk]

We are using dnn.rediscachingprovider for web farm. Have custom code that adds custom cache items with DataCache.SetCache(...). Had an issue because in some cases dnn.rediscachingprovider cleaned also these custom cache items from redis and that was unexpected. Updated dnn.rediscachingprovider code that is removing cache items to not remove all cache items from redis, but to behave similarly as DotNetNuke.Services.Cache.ClearCacheInternal method and remove a subset of cache items based on dnn prefix.