code: private boolean isRecent(ByteBuffer slabId) {
          UUID uuid = TimeUUIDSerializer.get().fromByteBuffer(slabId.duplicate());
          long age = System.currentTimeMillis() - TimeUUIDs.getTimeMillis(uuid);
          return age <= Constants.OPEN_SLAB_MARKER_TTL.toMillis();
      }
docs: Use the age of the slabId as a heuristic to determine when we should ignore the lack of the "open slab marker".
  <p>
  When a slab is first created, the 'manifest' row is written with "open=true" but no columns are written to the
  'slab' column family.  In that initial state, scanning the slab and finding no "open slab" marker does *not*
  mean we've found a stale slab that can be closed or deleted.  We could fix this by writing the "open slab"
  marker before writing the manifest (not in the same batch mutation).  But since the marker would only live for 20
  minutes without a followup write, simply checking the slabId age has exactly the same effect with fewer writes.