Fix deadlock on DeferredConfirmations

[SpencerTorres]

This PR improves the changes proposed in #44 originally made by @shifengbin

Changes:

• Fixed deadlock that occurs with DeferredConfirmations when connection is reset
• Made the fix's code more consistent with the other methods in the file
• Added some documentation for Wait() and other internal methods
• Added unit test for deferredConfirmations.Close()
• Added integration test to confirm that .Wait() will return ack: false when connection is closed (as suggested by @DanielePalaia )

The newly added unit & integration tests both passed consistently when I ran them

This problem is also mentioned in #46

[DanielePalaia]

Hi thanks for your PR,

it looks good to me but in the integration test the PublishWithDeferredConfirm should publish to a non existent exchange in order to let Wait to return a false ack.

Can you replace line 1829 of integration_test.go to publish to a non existent exchange?

[SpencerTorres]

It passed for both scenarios interestingly enough. Updated it anyway to publish to an exchange that does not exist

[lukebakken]

Thank you!

[DanielePalaia]

Looks good! Approved!