protocol on_error called twice for the same issue

[progovoy]

Steps to reproduce on aioamqp 0.8.2:

• Just kill rabbitmq and you'll see that the error callback is called twice

Did not investigated it but, this happens because _close_channels in protocol.py is called on connection lost and on AmqpClosedConncetion exception.

Desired behavior: on connection error I would expect my callback to be called once to be able to handle reconnection properly.

[RemiCardona]

This will be dealt with as part of the API redesign of #118. This callback (like most/all others) is most likely going to be removed altogether.