STABLE: use a bit set instead of a hash set for votes

Currently, the votes hashset is cleared when the digest is reset, and then votes are added as STABLE messages come in. However, since the digest is based on an Address[] membership, we could create a FixedSizeBitSet, and the bit to set when a STABLE msg is received is computed as the sender's rank in that membership. When all bits are set, we can send a STABILITY message (= all votes have been received).
This takes up less space than a hashset.