Move serial values inside cached value [5.3.0-B1]

In Cache not reset in case of serial version loss [5.2.1] I've explained serial concept. This concept has major flaw: serial versions are stored as part of a key and not as part of cached data. Because of this 2 parallel requests to cached data will be building it's new version and create extra load on database.

Instead we should store current serial values as part of cache itself. Then we can always return stale cache value to other requests while 1st request will be building new cache value. Original idea from http://habrahabr.ru/post/43539/

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