Use same strategy for both client-side sharding and redis-cluster.

This change is possible because, after a few changes in redis-cluster,
our default cluster strategy used for client side sharding and the one
used for redis-cluster turned out to be exactly the same, except for
the hashing function used to calculate distribution.

Actually some checks used to enforce correctness are redundant in the
context of redis-cluster (e.g. the one used to make sure that keys in
requests performing cross-keys operations will hash to the same slot,
which is performed by the server) so we could also add a more dumb and
permissive cluster strategy that relies on checks performed by Redis.

The only difference between the client-side sharding strategy and the
one used for redis-cluster, aside from the hash function, is in the
rules used for extracting hash tags from keys since empty tags {} are
considered valid by redis-cluster. In v1.0.0 the strategy used for our
client-side sharding cluster will reflect this change, but we will not
change it in v0.8 since it can be considered a breaking changes as it
can potentially affect existing cluster deployments.
Update CHANGELOG