Added support for Guru Mediation, and raising a guru mediation if refCount <= 0 but decrRefCount is called against such an object

make sure that cmpobj is in decoded form when sorting by ALPHA (this solves edge case from previous commit where (!sortby && alpha) == 1)

set refcount of string objects retrieved from zipmaps to 0, so we don't have to touch the refcount of the objects inside dicts

Incrementally rehahsing hash table! Thanks to Derek Collison and Pieter Noordhuis for feedbacks/help

Now all the commands returning a multi bulk reply against non existing keys will return an empty multi bulk, not a nil one