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).
does not work): Changing packed row format to only include null bits
for those columns that are present in the row as well as writing BIT
columns in a storage engine-independent format.
The change in row format is incompatible with the previous format and a
slave will not be able to read the new events.
