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.