R1:d67b7f0f47c4

Correct column mutations for old versions of MySQL

Summary:
Ref T1191. Although I fixed some of the mutations earlier (in D10598), I missed the column mutations under old versions of MySQL. In particular, this isn't valid:

- `ALTER TABLE ... MODIFY columnName VARCHAR(64) COLLATE binary`

Issue the permitted version of this instead, which is:

- `ALTER TABLE ... MODIFY columnName VARBINARY(64)`

Also fixed an issue where a clean schema had the wrong nullability for a column in the draft table. Force…