Implement ADD COLUMN on non empty spaces

[romanhabibov]

We need to implement a mechanism for filling in the DEFAULT, NULL, NOT NULL columns at the box (?) level.

[pgulutzan]

See also Issue #3075 Enable addition of new columns in ALTER

[capr]

wait so if I wanna add a column to a table currently I have to create a new table with the added column, copy all the data over, drop the old table along with all the foreign key pointing to it, rename the new table, re-add the foreign keys?

[capr]

Wait, I can't re-add foreign keys because the other tables have data too in them.

So currently in Tarantool you cannot do any changes to the schema of an existing database at all?