[RHM-1969] Gloat should support rollback of migrations.

[ndyakov]

For rhyme to be able to rollback to a prior version our migrations library was forked and additional features were implemented.
There are two possible ways for reverting migrations between versions:

1. If both version are on the same base migrate to <version> can be used. In that case migrations applied after <version> will be reverted.

• to get the latest migration that is part of a given version, there is migrate latest command.
• for correct ordering of migrations applied_at field is being added to the database.

3. If the two versions are not part of the same node and their migrations may be applied out of order, the migrate keep <list,of,versions,to,keep> can be used. In that case, all migrations listed (in the comma separated list) will be kept, the rest that are applied will be reverted.

• to generate the list of migrations that are part of a given version, there is migrate present command.
• for correct ordering of migrations applied_at field is being added to the database.

To deploy this feature, the changes of the schema_migrations table should be executed manually:

ALTER TABLE schema_migrations 
ADD COLUMN applied_at timestamp without time zone default (now() at time zone 'utc');

CREATE INDEX IF NOT EXISTS schema_migrations_applied_at 
ON schema_migrations (applied_at);

https://coursera.atlassian.net/browse/RHM-1969

TODO:

• tests
• update help
• write deployment steps

[borisbsv]

We use https://pkg.go.dev/github.com/stretchr/testify/assert for assertions, let's drop gsamokovarov/assert

[borisbsv]

By convention the name toCmd looks like it should convert the arguments to a Cmd, can we improve the naming a bit?

[borisbsv]

Let's just in case, use time.Now().UTC() for consistency across platforms?

[borisbsv]

Let's delete this variable it's only used in a single function and is initialized every time you load the package.

[borisbsv]

Do we sort here? If not, we implicitly rely on the db's internals and that's not guaranteed to be stable - you might append migrations that were applied before the one you're looking for

[borisbsv]

the variables m, migrations and intersect aren't super clear here, can we rename migrations to others or intersectWith and intersect with result?
Side note, using named returns isn't a good practice unless necessary.

[Hepri]

Looks good

[ndyakov]

@borisbsv can you check this PR again.

[borisbsv]

Merge on :)