Allow running certain migrations outside of a transaction.

[jwoertink]

Fixes #1129
Closes #1021

This allows you to specify that your migration is going to potentially run some "unsafe" code which basically just means that you're going to run it outside of a transaction. This can be used for when engines like Cockroach DB require running outside of a migration, or for things like creating indexes concurrently in postgres.

You just add the unsafe_migration call to the top of the migration.

class MyUnsafeMigration::V1234 < Avram::Migrator::Migration::V1
  unsafe_migration

  def migrate
    execute "some sql that won't run inside of a transaction"
  end

  def rollback
    execute "a rollback that also can't run inside of a transaction"
  end
end