feat: no tx migration

[cleverjam]

Does your PR solve an issue?

Yes, this change allows for a way to create migrations that will be executed outside of the postgres transaction block.

It works as described in this comment

fixes #767

😬 accidentally closed #3159 when looking at merge conflicts

[abonander]

One last nit.

[cleverjam]

could someone help me figure out why this test is failing? I can't reproduce the failure locally with DATABASE_URL=postgres://postgres:password@localhost:55595/sqlx cargo test --no-default-features --features any,postgres,macros,_unstable-all-types,runtime-async-std,tls-native-tls

Where 0.0.0.0:55595->5432/tcp sqlx-postgres_11-1

Edit: it seems this test passed previous builds fine (failures were due to formatting), should we re-run?

[cleverjam]

@abonander I have addressed your comment, please take a look when you have a chance

[qrilka]

@cleverjam @abonander is it expected that there seem to be no documentation update in this PR?

[abonander]

I generally don't require PR authors to write documentation because I usually end up wanting to edit it heavily anyway.

Migrations need a documentation pass in general, they're barely covered at all to begin with. When I get to it, I'll make sure to mention this feature.