Add a `with_lock` method to ActiveRecord objects, which starts
a transaction, locks the object (pessimistically) and yields to the block.
The method takes one (optional) parameter and passes it to `lock!`.

Before:

    class Order < ActiveRecord::Base
      def cancel!
        transaction do
          lock!
          # ... cancelling logic
        end
      end
    end

After:

    class Order < ActiveRecord::Base
      def cancel!
        with_lock do
          # ... cancelling logic
        end
      end
    end