Add beforeAll/afterAll events + file locking

[mmkal]

Follow-up to #394

This adds beforeAll and afterAll events. They run before the first and after the last migration respectively, for both up and down calls. This allows for locking functionality as requested in #144. Right now, there's a FileLocker class which uses the filesystem to implement a very basic locking mechanism. Adding actual database locking is more complex though, since every database system implements locks differently. I plan to use this to add locking to @slonik/migrator, since that's postgresql-only. It might be possible to add a SequelizeLocker class similar to FileLocker using transactions, but if that's difficult it could come in another follow-up. @papb have you use sequelize locks before?

[papb]

Very nice! Great idea. As for using Sequelize transactions to lock the table, that should work, and shouldn't be too hard. I don't have time to try to do it though... Not now at least.