An user raised a question about a given behavior of PFCOUNT. Added a
test to show the behavior (union) is correct when most of the items are
in common.

Make sure to flush the AOF output buffers before reloading.
Result: less false timing related false positives on AOF tests.

Currently this feature is only accessible via DEBUG for testing, since
otherwise depending on the instance configuration a given script works
or is broken, which is against the Redis philosophy.

This commit also inverts two stanzas of the code just becuase they are
more logical like that, not because currently it makes any difference.

By calling redis.replicate_commands(), the scripting engine of Redis
switches to commands replication instead of replicating whole scripts.
This is useful when the script execution is costly but only results in a
few writes performed to the dataset.

Morover, in this mode, it is possible to call functions with side
effects freely, since the script execution does not need to be
deterministic: anyway we'll capture the outcome from the point of view
of changes to the dataset.

In this mode math.random() returns different sequences at every call.

If redis.replicate_commnads() is not called before any other write, the
command returns false and sticks to whole scripts replication instead.

Sometimes it can be useful for clients to completely disable replies
from the Redis server. For example when the client sends fire and forget
commands or performs a mass loading of data, or in caching contexts
where new data is streamed constantly. In such contexts to use server
time and bandwidth in order to send back replies to clients, which are
going to be ignored, is a shame.

Multiple mechanisms are possible to implement such a feature. For
example it could be a feature of MULTI/EXEC, or a command prefix
such as "NOREPLY SADD myset foo", or a different mechanism that allows
to switch on/off requests using the CLIENT command.

The MULTI/EXEC approach has the problem that transactions are not
strictly part of the no-reply semantics, and if we want to insert a lot
of data in a bulk way, creating a huge MULTI/EXEC transaction in the
server memory is bad.

The prefix is the best in this specific use case since it does not allow
desynchronizations, and is pretty clear semantically. However Redis
internals and client libraries are not prepared to handle this
currently.

So the implementation uses the CLIENT command, providing a new REPLY
subcommand with three options:

    CLIENT REPLY OFF disables the replies, and does not reply itself.
    CLIENT REPLY ON re-enables the replies, replying +OK.
    CLIENT REPLY SKIP only discards the reply of the next command, and
                      like OFF does not reply anything itself.

The reason to add the SKIP command is that it allows to have an easy
way to send conceptually "single" commands that don't need a reply
as the sum of two pipelined commands:

    CLIENT REPLY SKIP
    SET key value

Note that CLIENT REPLY ON replies with +OK so it should be used when
sending multiple commands that don't need a reply. However since it
replies with +OK the client can check that the connection is still
active and all the previous commands were received.

This is currently just into Redis "unstable" so the proposal can be
modified or abandoned based on users inputs.

Add copy & replace parameters to redis-trib import

This new function is able to restart the server "in place". The current
Redis process executes the same executable it was executed with, using
the same arguments and configuration file.

Kinda related to #2770.

the check for lat/long valid ranges were performed inside the for loop,
two times instead of one, and the first time when the second element of
the array, xy[1], was yet not populated. This resulted into issue #2799.

Close issue #2799.

Now way to make unmodified Jemalloc configure to work when the jemalloc
source tree is inside a subdirectory of a different git repository.

Problem signaled here:
http://www.canonware.com/pipermail/jemalloc-discuss/2015-October/001174.html