Variadic write commands support

[nike-17]

Finally many write commands are able to take multiple values! This is the full list:

SADD set val1 val2 val3 ... -- now returns the number of elements added (not already present).
HDEL hash field2 field3 field3 ... -- now returns the number of elements removed.
SREM set val1 val2 val3 ... -- now returns the number of elements removed.
ZREM zset val1 val2 val3 ... -- now returns the number of elements removed.
ZADD zset score1 val1 score2 val2 ... -- now returns the number of elements added.
LPUSH/RLPUSH list val1 val2 val3 ... -- return value is the new length of the list, as usually.

Since Redis ability to process commands faster is not usually related to the time needed to alter the data set, but to the time spent into I/O, dispatching, sending the reply back, this means that now for some applications there is some impressive speed improvement.