This patch adds the ability to store extra field metadata in the table
map event. This data can include pack_length() or field_lenght() for
fields such as CHAR or VARCHAR enabling developers to add code that
can check for compatibilty between master and slave columns. More 
importantly, the extra field metadata can be used to store data from the
master correctly should a VARCHAR field on the master be <= 255 bytes 
while the same field on the slave is > 255 bytes. '

The patch also includes the needed changes to unpack to ensure that data
which is smaller on the master can be unpacked correctly on the slave.