This also optimizes ZREVRANGEBYSCORE for pathological cases where a
sorted set contains many elements with the same score. Previously,
it would traverse the list from back to front in such a case.

fix type in rdbSaveKeyValuePair() when saving an intset. Don't merge this commit into 2.2 as will not apply cleanly.

save ziplist encoded type as a different type id. Done as separated commit since this is the part that will not merge cleanly in 2.2