Write index marker i.

def _MaybeWriteIndex(self, i, ts, mutation_pool):
    """Write index marker i."""
    if i > self._max_indexed and i % self.INDEX_SPACING == 0:
      # We only write the index if the timestamp is more than 5 minutes in the
      # past: hacky defense against a late write changing the count.
      if ts[0] < (rdfvalue.RDFDatetime.Now() -
                  self.INDEX_WRITE_DELAY).AsMicrosecondsSinceEpoch():
        mutation_pool.CollectionAddIndex(self.collection_id, i, ts[0], ts[1])
        self._index[i] = ts
        self._max_indexed = max(i, self._max_indexed)