When we fail to setup the write handler it does not make sense to take
the client around, it is missing writes: whatever is a client or a slave
anyway the connection should terminated ASAP.

Moreover what the function does exactly with its return value, and in
which case the write handler is installed on the socket, was not clear,
so the functions comment are improved to make the goals of the function
more obvious.

Also related to #2485.

This should likely fix a false positive when running with the --valgrind
option.

master was closing the connection if the RDB transfer took long time.
and also sent PINGs to the slave before it got the initial ACK, in which case the slave wouldn't be able to find the EOF marker.

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.

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.

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.

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.

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.

Related to issue #2372.

Fixing #2371 as per @mattsta's suggestion

Backtrace is a glibc extension, while setproctitle() implementation
depends on the memory layout and is partially libc dependent.

Less grays: more readable palette since usually we have a non linear
distribution of percentages and very near gray tones are hard to take
apart. Final part of the palette is gradient from yellow to red. The red
part is hardly reached because of usual distribution of latencies, but
shows up mainly when latencies are very high because of the logarithmic
scale, this is coherent to what people expect: red = bad.

When trying to debug sentinel connections or max connections errors it
would be very useful to have the ability to see the list of connected
clients to a running sentinel. At the same time it would be very helpful
to be able to name each sentinel connection or kill offending clients.

This commits adds the already defined CLIENT commands back to Redis
Sentinel.

Itereator misuse due to analyzeLatencyForEvent() accessing the
dictionary during the iteration, without the iterator being
reclared as safe.