make the readCombineCas retries re-read between failures

Currently it does this: 

  get old value 
  compute new value as (old val + update)
  for 1 .. casRetries
     try to checkAndPut(.., old val, new value) 

if the value in the table changes, this will fail between reading and updating. Retrying won't help, because we don't read a new old value. This change helps that by rereading the old value between retries.