This command is equal to SUNION, but instead of returning the resulting set,
it is stored in destination.
If destination already exists, it is overwritten.
@return
@integer-reply: the number of elements in the resulting set.
@examples
SADD key1 "a"
SADD key1 "b"
SADD key1 "c"
SADD key2 "c"
SADD key2 "d"
SADD key2 "e"
SUNIONSTORE key key1 key2
SMEMBERS key