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.

1. Remove useless "cs" initialization.
2. Add a "select" var to capture a condition checked multiple times.
3. Avoid duplication of the same if (!copy) conditional.
4. Don't increment dirty if copy is given (no deletion is performed),
   otherwise we propagate MIGRATE when not needed.

Add last_dbid to migrateCachedSocket to avoid redundant SELECT

Avoid redundant SELECT calls when continuously migrating keys to
the same dbid within a target Redis instance.

Related to PR #2357.

Related to PR #2357.

Signed-off-by: NChris Lamb <chris@chris-lamb.co.uk>