Set channels and allocator to nil in shutdown

[lukebakken]

Follow up to #171

cc @Zerpet

cc @jxsl13 since it may be interesting

[jxsl13]

will run some tests tomorrow to see how this behaves.

[jxsl13]

I think due to the slash in the branch name, it's not trivially possible to go get this branch as dependency:

go: github.com/rabbitmq/amqp091-go@lukebakken/nil-for-cleanup: invalid version: version "lukebakken/nil-for-cleanup" invalid: disallowed version string

Edit: using the hash did the job:

go get github.com/rabbitmq/amqp091-go@2d5555937d87f8c64e4d19a76ac6c8d0f4d7c77d
go: downloading github.com/rabbitmq/amqp091-go v1.6.2-0.20230208180231-2d5555937d87
go: upgraded github.com/rabbitmq/amqp091-go v1.6.2-0.20230208132010-0ecb414c61ad => v1.6.2-0.20230208180231-2d5555937d87

[jxsl13]

tests still work.

[Zerpet]

I added a test case to ensure that Connection.Channel() won't panic, when called on a closed connection. A user may try to use a closed connection. In such case, the library should return an error (not panic).

[Zerpet]

@lukebakken happy with the code as is. Feel free to add your review to my test case before merging :)

[lukebakken]

@jxsl13 - I'll keep that in mind for future PRs (don't use a slash). Thanks for testing.

[jxsl13]

Thanks for maintaining the library :)

[Zerpet]

GitHub Actions is having a bad day and has queued the CodeQL action for 35 minutes (and counting). Given that previous commits passed the CI ✅ And that main is also green, I'm going to merge as-is.