`decide_t` should take a multiset.

Currently, decide functions take in a set of replica replies (see here). However, this prevents the decide function from knowing the counts of the replies. For example, the lock server decide function considers a lock acquired if a majority of replicas reply "0" indicating the lock was granted. However, if all but one replica replies "0" and one replica replies "-1" (indicating the lock was not acquired), then the set passed to the decide function is a set of just {"0", "1"}. This prevents the decide function from knowing how many replicas replied with "0".

Instead, decide functions should take in something like a multiset<string> or just a map<string, int> of counts.