DB Producer can fail if all messages in batch were unique

[colinmroberts]

In the DB producer, if a compacted topic has a batch with all-unique keys, it will fail when calling compact_messages with this error:

NoMethodError:
       undefined method `reverse!' for nil:NilClass

E.g. consider this sample test which should pass:

      it 'should no make any changes when there are no duplicates' do
        Deimos.configure { |c| c.db_producer.compact_topics = :all }
        expect(producer.compact_messages(deduped_batch)).to eq(deduped_batch)
      end

This appears to be because compact batch is calling reverse! on uniq!. uniq! will return nil when no changes were made to the array, i.e. it was unique already. This method should use uniq instead