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.