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 such
a case, this bug could happen.

This change to ae.c modifies the event loop implementation to make this
concept explicit. Write events that are registered with:

    AE_WRITABLE|AE_BARRIER

Are guaranteed to never fire after the readable event was fired for the
same file descriptor. In this way we are sure that data is persisted to
disk before the client performing the operation receives an
acknowledged.

However note that this semantics does not provide all the guarantees
that one may believe are automatically provided. Take the example of the
blocking list operations in Redis.

With AOF and fsync=always we could have:

    Client A doing: BLPOP myqueue 0
    Client B doing: RPUSH myqueue a b c

In this scenario, Client A will get the "a" elements immediately after
the Client B RPUSH will be executed, even before the operation is persisted.
However when Client B will get the acknowledge, it can be sure that
"b,c" are already safe on disk inside the list.

What to note here is that it cannot be assumed that Client A receiving
the element is a guaranteed that the operation succeeded from the point
of view of Client B.

This is due to the fact that the barrier exists within the same socket,
and not between different sockets. However in the case above, the
element "a" was not going to be persisted regardless, so it is a pretty
synthetic argument.