Enable automatic connection recovery by default

Automatic connection recovery has been around for over 2 years in this client and some 5 years total (starting with the amqp gem in Ruby, then Bunny). With a round of fixes up to and in `3.6.6` we know that it works quite well in practice and what kind of bugs and tricky aspects we've discovered over the years. It's time to enable it by default and `4.0.0` is the right moment to do it.
