private void tryRetireChunk(Chunk c) {
    currChunk.compareAndSet(c, null);
    // If the CAS succeeds, that means that we won the race
    // to retire the chunk. We could use this opportunity to
    // update metrics on external fragmentation.
    //
    // If the CAS fails, that means that someone else already
    // retired the chunk for us.
  }
Try to retire the current chunk if it is still <code>c</code>. Postcondition is that
curChunk.get() != c
@param c the chunk to retire
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreLABImpl.java