Cluster: new sub-command CLUSTER COUNTKEYSINSLOT.

The new sub-command uses the new countKeysInSlot() API and allows a
cluster client to get the number of keys for a given hashslot.
Cluster: use O(log(N)) algo for countKeysInSlot().
Cluster: validate slot number in CLUSTER COUNTKEYSINSLOT.
Cluster: no limits for the count parameter of CLUSTER GETKEYSINSLOT.

Not sure why I set a limit to 1 million keys, there is no reason for
this artificial limit, and anyway this is s a stupid limit because it is
already high enough to create latency issues. So let's the users shoot
on their feet because maybe they just actually know what they are doing.
Fix error "repl-backlog-size must be 1 or greater"

The parameter repl-backlog-size is not parsed correctly in the configuration file. argv[0] is parsed instead of argv[1].
Merge pull request #969 from serphen/unstable

Fix error "repl-backlog-size must be 1 or greater"
Cluster: new function clusterNodeDelFailureReport() for failure reports.

This is the missing part of the API that will be used to reimplement
failure detection of Cluster nodes.