Add support for instant add column for InnoDB when adding a new column with a default value last.
We have got patches from Alibaba and TenCent for doing this. We should choose either one or merge them.
The idea in both patches is to add a new row type to InnoDB where the number of columns
are stored in each row in 1-2 bytes. When creating a row we store the current number of columns. This allows us to fill any extra columns with default values on read.