function makeBloomFilter(backend, lastSync) {
  const newChanges = Backend.getChanges(backend, lastSync)
  const hashes = newChanges.map(change => decodeChangeMeta(change, true).hash)
  return { lastSync, bloom: new BloomFilter(hashes).bytes }
}
Constructs a Bloom filter containing all changes that are not one of the hashes in
`lastSync` or its transitive dependencies. In other words, the filter contains those
changes that have been applied since the version identified by `lastSync`. Returns
an object of the form `{lastSync, bloom}` as required for the `have` field of a sync
message.