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.

Differently to v0.8, the strategy for client-side sharding now uses
the same rules for extracting hash tags from keys especially when
empty tags are found in the string.