Free format rollback tags

[cloend]

Would it be possible to have the rollbackVersion field accept alphanumerical values instead of an integer?
Or perhaps to keep it backwards compatible add a tag or prefix field, which value is added to the rollbackVersion when writing to the databasechangelog table. The default value would be a 'v' as is the implementation at the moment.

Because we are running into the following problem. We have multiple applications that use Liquibase to keep track of the database objects, only they all share the same schema. So each application will be writing in the databasechangelog. Each application would get a v1 tag, which on rollbacks results in misbehavior by Liquibase.

@hbogaards already made the same sort of change hbogaards@58624c5