Fix OBJECT arity

Previously, the command definition for the OBJECT command specified
a minimum of two args (and that it was variadic), which meant that
if you sent this:

OBJECT foo

When cluster was enabled, it would result in an assertion/SEGFAULT
when Redis was attempting to extract keys.

It appears that OBJECT is not variadic, and only ever takes 3 args.

https://gist.github.com/michael-grunder/25960ce1508396d0d36a
Merge pull request #1870 from michael-grunder/object_arity

Fix OBJECT arity
RDB: load string objects directly as EMBSTR objects when possible.
PubSub clients refactoring and new PUBSUB flag.

The code tested many times if a client had active Pub/Sub subscriptions
by checking the length of a list and dictionary where the patterns and
channels are stored. This was substituted with a client flag called
REDIS_PUBSUB that is simpler to test for. Moreover in order to manage
this flag some code was refactored.

This commit is believed to have no effects in the behavior of the
server.
Variadic PING with support for Pub/Sub.

PING can now be called with an additional arugment, behaving exactly
like the ECHO command. PING can now also be called in Pub/Sub mode (with
one more more subscriptions to channels / patterns) in order to trigger
the delivery of an asynchronous pong message with the optional payload.

This fixes issue #420.
tryObjectEncoding(): use shared objects with maxmemory and non-LRU policy.

In order to make sure every object has its own private LRU counter, when
maxmemory is enabled tryObjectEncoding() does not use the pool of shared
integers. However when the policy is not LRU-based, it does not make
sense to do so, and it is much better to save memory using shared
integers.
PING: backward compatible error for wrong number of args.
Test: small integer sharing depends on maxmemory policy.
Test: Pub/Sub PING.
Cluster test: test that writes are retained during failovers.

In the test we use WAIT when the master and slave are up, and only later the
partition is created killing the master, so we are sure we don't incur
in failure modes that may lose writes in this test: the goal here is to
make sure that the elected slave was replicating correctly with the
master.
redis-trib: allow to reshard in non-interactive way.

The introduction of --from --to --slots --yes options allow to reshard
from cli in an automated way from scripts. The code is ugly and needs
refactoring as soon as we get it in RC / stable release.
Cluster: fix redis-trib --from all.
Cluster test: unit 04, consistency during resharding.
Cluster test: use larger keyspace in resharding test.
CLUSTER RESET: Flush dataset if node is a slave.

For non-empty masters, CLUSTER RESET is denied, and the user requires to
start to reset a node by explicitly clearing it with FLUSHALL.
However CLUSTER RESET when executed with slaves don't have this
restrictions since data is just a replica of the master, and with
read-only slaves it is also not possible to remove the data set. However
the node was turned from slave to master after a reset, without touching
the old slave data. This is 99.99% of times not appropriate and forces
full resets to follow this path to work with both slave and master
nodes:

    FLUSHALL
    CLUSTER RESET HARD
    FLUSHALL

Since we need the first flushall for masters, and the second for slaves.

This commit changes the behavior so that CLUSTER RESET removes the data set
of a slave node during a reset, in the moment it gets turned into a master,
so the new pattern is simply:

    FLUSHALL (that may fail for slaves)
    CLUSTER RESET
Add double field in dict.c entry value union.
ZUNIONSTORE reimplemented for speed.

The user @kjmph provided excellent ideas to improve speed of ZUNIONSTORE
(in certain cases by many order of magnitude), together with an
implementation of the ideas.

While the ideas were sounding, the implementation could be improved both
in terms of speed and clearness, so that's my attempt at reimplementing
the speedup proposed, trying to improve by directly using just a
dictionary with an embedded score inside, and reusing the single-pass
aggregate + order-later approach.

Note that you can't apply this commit without applying the previous
commit in this branch that adds a double in the dictEntry value union.

Issue #1786.
Remove useless var and check in zunionInterGenericCommand().
Test: check sorted set elements order after union.

This is not a regression but issue #1786 showed the need for this test.
PFSELFTEST: less false positives.

This is just a quickfix, for the nature of the test the right way to fix
it is to average the error of N runs, since otherwise it is always
possible to get a false positive with a bad run, or to minimize too much
this possibility we may end testing with too much "large" error ranges.