ae.c: introduce the concept of read->write barrier.
AOF fsync=always, and certain Redis Cluster bus operations, require to fsync data on disk before replying with an acknowledge. In such case, in order to implement Group Commits, we want to be sure that queries that are read in a given cycle of the event loop, are never served to clients in the same event loop iteration. This way, by using the event loop "before sleep" callback, we can fsync the information just one time before returning into the event loop for the next cycle. This is much more efficient compared to calling fsync() multiple times. Unfortunately because of a bug, this was not always guaranteed: the actual way the events are installed was the sole thing that could control. Normally this problem is hard to trigger when AOF is enabled with fsync=always, because we try to flush the output buffers to the socekt directly in the beforeSleep() function of Redis. However if the output buffers are full, we actually install a write event, and in suc...
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