Cluster: added new API countKeysInSlot().

This is similar to getKeysInSlot() but just returns the number of keys
found in a given hash slot, without returning the keys.
Cluster: added stub for verifyClusterConfigWithData().

See the top-comment for the function in this commit for details about
what the function is supposed to do.
Cluster: use CountKeysInSlot() when we just need the count.
Cluster: fix case for getKeysInSlot() and countKeysInSlot().

Redis functions start in low case. A few functions about cluster were
capitalized the wrong way.
A comment in main() clarified.
Cluster: verifyClusterConfigWithData() implemented.