Proposal: use uuid instead of numbers to identify migration steps

[guillep2k]

Migrations could have a uuid instead of a number, and the 'version' table should just have the list of migrations already done. The order in which the migrations are performed is important, of course, but that is already determined by the order of the migrations slice in migrations.go.

So, backporting a migration or merging a PR into master will hardly ever produce a conflict except when one migration is dependent of another (but even that case can be covered, for instance):

func NewMigration(desc string, fn func(*xorm.Engine) error, uuid string, dependsOn []string) Migration {
}

If a migration signals another as a dependency and that one migration was not yet merged (e.g. when backporting), the integration tests will simply fail.