Don't reconfirm already acked messages when multiple is true

[kinnalru]

In current implementation library acks all tags starting from tag 1 (plus channel recovery offset) every time it gets multiple flag regardless of previously acknowledged tags. So when multiple flag occured ex. at delivery_tag 1000 (after 990 aked), we receive confirm_select callback 1000 times. And we receive this call callback again and again (hudren thousands times) while processing just 16k messages.

After some research of other language implementation (java and python) we made a little fix to resolve this issue. RabbitMQ protocol extension strgongly enforces sequenced delivery_tag generation on publishing. There is @unconfirmed_set filled with not-yet-acked tags (previously acked tags already removed from it) so we just need to ack tags from first (smallest) unconfirmed position till currentlry delivered with multiple flag to fit RabbitMQ publisher confirms protocol extension.

There is additional test case to test and compare old and new behaviour. It is not for enclusion in resulting PR - just for experiments. You can change messages count to higher value when difference increased drastically.

Fixes #615

[michaelklishin]

Thank you for the thorough PR. Note that you can add benchmarks under benchmarks, e.g. we can keep the one that tests the proposed implementation there. Or we can drop it as it's served its purpose.

My test runs on a 4-year old CPU suggest that once the number of messages published goes above 20K or so, the new implementation begins significantly outperforming. In general, this implementation should be acceptable
for most users even if Set#min is not optimised to be a constant time operation.

So I'd be happy to merge once you remove the temporary tests (or move one of them to ./benchmarks). Thanks again for
putting in the time and effort 👏

[krappaU]

I deleted those specs, they've been created only for this PR, to show difference in performance. Thanks for your time and quick response!
P.S. Sorry for extra pr I made by mistake.