Possibility to use migrations in parallel feature branches

During executing migrations store not only the latest migration timestamp, but all timestamps of executed migrations. It allows to execute all migrations after merging two or more feature branches with migrations which were developed at the same time.

Before use you should import into propel_migration table timestamps of all already executed migrations.

New migration system is disabled by default and can be enabled using option:
```ini
propel.migration.parallel = true
```