[WIP] cmd/atomic-migrations-test: Atomic migrations test command
Created by: tsenart
This change set introduces:
- An isolated atomic migrations test command that reliably reproduces https://github.com/golang-migrate/migrate/issues/55 and https://github.com/sourcegraph/sourcegraph/issues/1491.
- A patch to
db/dbconn.go
that safely ignores the error we're seeing and thus fixes #1491. See the code comment for more details.
Since I shared this PR in https://github.com/golang-migrate/migrate/issues/55 with the test case, I'd like to keep the atomic-migrations-test
code around. If there's a better place for it, please advice.