We don't write the output buffer to the client socket for slaves only if
the slave is not online.

Should be "continue" and was "return".

This fixes issue #1110

Required because of recent changes in the way logfile is set to standard
output.

Also the logfile option was modified to always have an explicit value
and to log to stdout when an empty string is used as log file.

Previously there was special handling of the string "stdout" that set
the logfile to NULL, this always required some special handling.

The test checks both successful syncs and unsuccessful ones by changing
the backlog size.

The test actually worked, but vars for master and slave were inverted
and sometimes used incorrectly.

This reverts commit 2c75f2cf.

After further analysis, it is very unlikely that we'll raise the
string size limit to > 512MB, and at the same time such big strings
will be used in 32 bit systems.

Better to revert to size_t so that 32 bit processors will not be
forced to use a 64 bit counter in normal operations, that is currently
completely useless.

in 32bit machine, popcount don't work with a input string length up to 5...

bitcount commant may return negtive integer with string length more than 256 MB

When the PONG delay is half the cluster node timeout, the link gets
disconnected (and later automatically reconnected) in order to ensure
that it's not just a dead connection issue.

However this operation is only performed if the link is old enough, in
order to avoid to disconnect the same link again and again (and among
the other problems, never receive the PONG because of that).

Note: when the link is reconnected, the 'ping_sent' field is not updated
even if a new ping is sent using the new connection, so we can still
reliably detect a node ping timeout.

Also clusterBroadcastPing() was renamed into clusterBroadcastPong()
that's what the function is actually doing.

This is just to make the code exactly like the above instance used for
requirepass. No actual change nor the original code violated the Redis
coding style.

There was a race condition in the AOF rewrite code that, with bad enough
timing, could cause a volatile key just about to expire to be turned
into a non-volatile key. The bug was never reported to cause actualy
issues, but was found analytically by an user in the Redis mailing list:

https://groups.google.com/forum/?fromgroups=#!topic/redis-db/Kvh2FAGK4Uk

This commit fixes issue #1079.