redis-trib: specify single node address when fixing coverage.
Cluster: empty the internal sorted set mapping keys to slots on FLUSHDB/ALL.
Cluster: fix case of slotToKey...() functions.
redis-trib: handle slot coverage fix in the "no nodes with keys" case.
redis-trib: skeleton of coverage fix for "keys in multiple nodes" case.
redis-trib: check that all the nodes agree about the slots configuration.
Cluster: introduced a failure reports system.

A §Redis Cluster node used to mark a node as failing when itself
detected a failure for that node, and a single acknowledge was received
about the possible failure state.

The new API will be used in order to possible to require that N other
nodes have a PFAIL or FAIL state for a given node for a node to set it
as failing.
Cluster: remove warning adding clusterNodeSetSlotBit() prototype.
Cluster: new state information, cluster size.

The definition of cluster size is: the number of known nodes in the
cluster that are masters and serving at least an hash slot.
Cluster: add cluster_size field in CLUSTER INFO output.
Cluster: if no previous config exists, create the myself node as master.