Multiplexed Commands Del, MSet, SInter and SUnion

Implementing multiplexed commands (Del, MSet, SInter and SUnion as well as SAdd, which is needed to test SInter and SUnion).

Tests with server done.
Sinter command returns a set that is an intersection of all the sets asked through the keys. If even one of them is an empty set, the returned set will also be empty. So there is no point in continuing in the loop for x times. You just return an MultiBulkReply that has an empty set as value.

from the redis doc: 
Keys that do not exist are considered to be empty sets. With one of the keys being an empty set, the resulting set is also empty (since set intersection with an empty set always results in an empty set).