No semantical changes since to make dict.c truly able to scale over the
32 bit table size limit, the hash function shoulds and other internals
related to hash function output should be 64 bit ready.

rehashidx is always positive in the two code paths, since the only
negative value it could have is -1 when there is no rehashing in
progress, and the condition is explicitly checked.

Bug as old as Redis and blocking operations. It's hard to trigger since
only happens on instance role switch, but the results are quite bad
since an inconsistency between master and slave is created.

How to trigger the bug is a good description of the bug itself.

1. Client does "BLPOP mylist 0" in master.
2. Master is turned into slave, that replicates from New-Master.
3. Client does "LPUSH mylist foo" in New-Master.
4. New-Master propagates write to slave.
5. Slave receives the LPUSH, the blocked client get served.

Now Master "mylist" key has "foo", Slave "mylist" key is empty.

Highlights:

* At step "2" above, the client remains attached, basically escaping any
  check performed during command dispatch: read only slave, in that case.
* At step "5" the slave (that was the master), serves the blocked client
  consuming a list element, which is not consumed on the master side.

This scenario is technically likely to happen during failovers, however
since Redis Sentinel already disconnects clients using the CLIENT
command when changing the role of the instance, the bug is avoided in
Sentinel deployments.

Closes #2473.

There was a bug in Redis Cluster caused by clients blocked in a blocking
list pop operation, for keys no longer handled by the instance, or
in a condition where the cluster became down after the client blocked.

A typical situation is:

1) BLPOP <somekey> 0
2) <somekey> hash slot is resharded to another master.

The client will block forever int this case.

A symmentrical non-cluster-specific bug happens when an instance is
turned from master to slave. In that case it is more serious since this
will desynchronize data between slaves and masters. This other bug was
discovered as a side effect of thinking about the bug explained and
fixed in this commit, but will be fixed in a separated commit.

create-cluster fix for stop and watch commands

1. No need to set btype in processUnblockedClients(), since clients
   flagged REDIS_UNBLOCKED should have it already cleared.
2. When putting clients in the unblocked clients list, clientsArePaused()
   should flag them with REDIS_UNBLOCKED. Not strictly needed with the
   current code but is more coherent.

When the list of unblocked clients were processed, btype was set to
blocking type none, but the client remained flagged with REDIS_BLOCKED.
When timeout is reached (or when the client disconnects), unblocking it
will trigger an assertion.

There is no need to process pending requests from blocked clients, so
now clientsArePaused() just avoid touching blocked clients.

Close #2467.

In no case we should try to attempt to failover if myself->slaveof is
NULL.

This commit moves the process of generating a new config epoch without
consensus out of the clusterCommand() implementation, in order to make
it reusable for other reasons (current target is to have a CLUSTER
FAILOVER option forcing the failover when no master majority is
reachable).

Moreover the commit moves other functions which are similarly related to
config epochs in a new logical section of the cluster.c file, just for
clarity.

Before we relied on the global cluster state to make sure all the hash
slots are linked to some node, when getNodeByQuery() is called. So
finding the hash slot unbound was checked with an assertion. However
this is fragile. The cluster state is often updated in the
clusterBeforeSleep() function, and not ASAP on state change, so it may
happen to process clients with a cluster state that is 'ok' but yet
certain hash slots set to NULL.

With this commit the condition is also checked in getNodeByQuery() and
reported with a identical error code of -CLUSTERDOWN but slightly
different error message so that we have more debugging clue in the
future.

Root cause of issue #2288.

Related to issue #2288.

There are rare conditions where node->slaveof may be NULL even if the
node is a slave. To check by flag is much more robust.

Support CLIENT commands in Redis Sentinel

If count is 0 SADD is called without element arguments, which is
currently invalid.

Was missing for some reason. Trivial to add after config.c refactoring.

Still many things to convert inside config.c in the next commits.
Some const safety in String objects creation and addReply() family
functions.

Add command CLUSTER MYID

Not perfect since The Solution IMHO is to have a DSL with a table of
configuration functions with type, limits, and aux functions to handle
the odd ones. However this hacky macro solution is already better and
forces to put limits in the range of numerical fields.

More field types to be refactored in the next commits hopefully.