title: Fill the "na" values with unique "na" identifier when doing pandas merge
body: <p>I want to merge two pandas dataframe.</p>
  
  <pre><code>df1 = 
  A   B
  2   11
  2   13
  2   15
  2   19
  2   25
  2   35
  2   41
  2   47
  2   46
  2   51
  3   9
  3   15
  3   17
  3   23
  3   25
  3   29
  5   4
  5   23
  5   28
  </code></pre>
  
  <p>with another dataframe. </p>
  
  <pre><code>   df2 = 
  A   B    C
  2   11   abc
  2   13   cdd
  2   35   cdd
  2   41   cdd
  2   47   cdd
  3   9   cdd
  3   15   cdd
  3   17   cdd
  3   23   cdd
  </code></pre>
  
  <p>Both dataframes are sorted by "A" and then "B". I want to merge by <code>columns['A', 'B']</code>; so for column "C" where the data are missing I want to fill them by <code>na</code>, but with <code>na_uniqueNumber</code> for each missing block of <code>na</code>.</p>
  
  <p><strong>How can I updated this merge method:</strong></p>
  
  <pre><code>data_frames = [df1, df2]
  df_update = reduce(lambda left,right: pd.merge(
      left, right, on=['A', 'B'], how='outer'), data_frames).fillna('na')
  </code></pre>
  
  <p><strong>Note:</strong> The code should update <code>na</code> with unique values only in "C" in the situation other column are present.</p>
  
  <p><strong>Expected output:</strong></p>
  
  <pre><code>   df2 = 
  A   B    C
  2   11   abc
  2   13   cdd
  2   15   na_01
  2   19   na_01 
  2   25   na_01  
  2   35   cdd
  2   41   cdd
  2   47   cdd
  2   46   na_02
  2   51   na_02
  3   9   cdd
  3   15   cdd
  3   17   cdd
  3   23   cdd
  3   25   na_03
  3   29   na_03
  5   4   na_04
  5   23   na_04
  5   28   na_04
  </code></pre>
  
  <p>Thanks,</p>