Cluster: clusterBlacklistExists() requires blacklist cleanup before lookup.
Cluster: clusterBlacklistAddNode() key lookup fixed.

We can't lookup by node->name that's not an SDS string but a plain C
array in the node structure.
Cluster: fix clusterBlacklistAddNode() by setting right expire time.

The hash table value should be set to now + 60 seconds otherwise it
expires immediately.
Cluster: use the node blacklist in CLUSTER FORGET.

CLUSTER FORGET is not useful if we can't remove a node from all the
nodes of our cluster because of the Gossip protocol that keeps adding
a given node to nodes where we already tried to remove it.

So now CLUSTER FORGET implements a nodes blacklist that is set and
checked by the Gossip section processing function. This way before a
node is re-added at least 60 seconds must elapse since the FORGET
execution.

This means that redis-trib has some time to remove a node from a whole
cluster. It is possible that in the future it will be uesful to raise
the 60 sec figure to something bigger.