Fix return type in docstring for queue_declare
Add test for transactional mode
Make queue_declare_ok_t package-level
Reorganize tests
basic_publish(): wait for publisher confirms if enabled
Replace kwarg with positional arg
Simplify drain_events()
Add comment
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Rewrite channel mode mechanism

`Channel.mode` now reflects the state of the channel mode at all times. Default
publisher confirm for new channels is disabled, because it adds complexity and
offers no advantages in return.
Remove old TODOs
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Update docstrings
Improve documentation

* Member order in docs is sorted by type, then alphanetically
* Docstrings are added for instance attributes
Add docstrings for instance attributes
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