Re: using index to speedup add not null constraints to a table

hi.

In v1 I didn't do the `git add` for newly created isolation test related files.
so the cfbot for isolation tests failed.

v1 with index:
create index t_idx_ab on t(a,b);
we cannot fast add a not-null constraint for column b.
with the attached v2 patch, now we can do that.

v2, isolation test also adds other session drop index scarenio.