Redis functions start in low case. A few functions about cluster were
capitalized the wrong way.

See the top-comment for the function in this commit for details about
what the function is supposed to do.

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

The definition of cluster size is: the number of known nodes in the
cluster that are masters and serving at least an hash slot.

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.

Also using this new flag the RESTORE-ASKING command was implemented that
will be used by MIGRATE.

This fixes issue #607.