Brocast a PONG to all node in cluster when role changed

[enjoy-binbin]

When a node role changes, we should brocast the change to notify other nodes.
For example, one primary and one replica, after a failover, the replica
became a new primary, the primary became a new replica.

And then we trigger a second cluster failover for the new replica, the
new replica will send a MFSTART to its primary, ie, the new primary.

But the new primary may reject the MFSTART due to this logic:

    } else if (type == CLUSTERMSG_TYPE_MFSTART) {
        /* This message is acceptable only if I'm a primary and the sender
         * is one of my replicas. */
        if (!sender || sender->replicaof != myself) return 1;

In the new primary views, sender is still a primary, and sender->replicaof
is NULL, so we will return. Then the manual failover timedout.

Another possibility is that other primaries refuse to vote after receiving
the FAILOVER_AUTH_REQUEST, since in their's views, sender is still a primary,
so it refuse to vote, and then manual failover timedout.

void clusterSendFailoverAuthIfNeeded(clusterNode *node, clusterMsg *request) {
    ...
        if (clusterNodeIsPrimary(node)) {
            serverLog(LL_WARNING, "Failover auth denied to %.40s (%s) for epoch %llu: it is a primary node", node->name,
                      node->human_nodename, (unsigned long long)requestCurrentEpoch);

The reason is that, currently, we only update the node->replicaof information
when we receive a PING/PONG from the sender. For details, see clusterProcessPacket.
Therefore, in some scenarios, such as clusters with many nodes and a large
cluster-ping-interval (that is, cluster-node-timeout), the role change of the node
will be very delayed.

Added a DEBUG DISABLE-CLUSTER-RANDOM-PING command, send cluster ping to a random
node every second (see clusterCron).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.76%. Comparing base (979f4c1) to head (8d993ad).
Report is 2 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1295      +/-   ##
============================================
+ Coverage     70.74%   70.76%   +0.02%     
============================================
  Files           116      116              
  Lines         63280    63285       +5     
============================================
+ Hits          44767    44784      +17     
+ Misses        18513    18501      -12     

Files with missing lines	Coverage Δ
src/cluster_legacy.c	86.74% <100.00%> (+0.22%)	⬆️
src/debug.c	53.17% <100.00%> (+0.12%)	⬆️
src/server.c	87.65% <100.00%> (+<0.01%)	⬆️
src/server.h	100.00% <ø> (ø)

... and 11 files with indirect coverage changes

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[madolson]

Overall I think sending out the broadcast of messages is fine and should be safe.