rows event):

Adding a after image COLS bitmap to Update_rows_log_event (for telling
what columns that are present in the after image of each row update).

Also fixing case where Rows_log_event length was not correctly computed
(happened when the number of columns in a table was more than 251). 


mysql-test/r/rpl_row_inexist_tbl.result:
  Result change.
sql/log_event.cc:
  Extending Rows_log_event with two new fields: m_bitbuf_ai and m_cols_ai.  These fields are only used for the Update_rows_log_event.
  Adding implementation of Update_rows_log_event destructor.
  Using new after image fields inside the Update_rows_log_event.
  Factoring out common constructor bodies into Update_rows_log_event::init()
  function.
  
  Fixing case where length of Rows_log_event was not correctly computed (for
  tables with more than 251 columns).
sql/log_event.h:
  Moving implementation of Rows_log_event::get_data_size() into .cc file.
  Adding Update_rows_log_event constructor accepting both before image
  and after image COLS vector.
  Adding Update_rows_log_vector destructor.
  Adding fields m_bitbuf_ai and m_cols_ai to Rows_log_event.
  Fixing is_valid() to look at m_cols_ai as well.