Add warning/log message on max value length exceeded

The cache contract (get/set/add) does not allow the implementation to provide feedback to the caller, that a value to be stored was too long, and skipped. As the size of data to be saved is unknown from an external point of view (due to picklig + zlib), memcache impl should provide some ways of reporting such usecases....
Thus adding warnings or a custom logger would be nice.
Thx