Handle realTag = 0 in RecoveryAwareChannelN

[caspermout]

Proposed Changes

See #395

When a connection is lost and recovered, and the last message from the previous connection is acked with multi=true, the result is a basicAck with tag=0 and multi=1, this will ack every outstanding message.

This is a problem if there are messages that should be nacked. For example when the rabbit connection is lost, it would seem reasonable that other systems are also unreachable (like a database).
If it somehow happens that the last few messages in the previous connection are successful and acked with multi ack. Then the other unhandled messages will be acked as well.

This change makes sure that this does not happen and only sends a ack with multi=true and tag=0 when that is actually what is requested.

I did not create or run any tests because I am having trouble installing all the required dependencies.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes issue When recovery is enabled more deliveries can be acked after recovery than the user expects #395)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

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• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

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[michaelklishin]

Reviewed together with @acogoluegnes, we have some extra comments that will be added on top but the idea is sound.

Thank you!