code: function sumShards(store, cb) {
    exports.lazy(store.metaDb.createValueStream(), cb)
      .map(function(stream) {
        return stream.Shards.filter(function(shard) {
          return shard.SequenceNumberRange.EndingSequenceNumber == null
        }).length
      })
      .sum(function(sum) { return cb(null, sum) })
  }
docs: Sum shards that haven't closed yet