io_uring improvements (error sets, completion cancellation, queued write retries)

[recursiveGecko]

There are a couple of things here, I'm happy to split them up into multiple PRs:

• Adds some common error codes for send/recv operations (e.g. Connection refused, timed out, connection reset)
• Adds a generic completion cancellation function that can be used to cancel any completion (tested with TCP recv operations)
• Improves handling of partial queued writes. Public Queued API should guarantee the ordering of writes. These changes ensure that partially completed writes are re-tried rather than discarded.

[penberg]

@recursiveGecko I think it's fine to have these in one pull request, but obviously it's really @mitchellh's preference that matters here.

[mitchellh]

This looks really good. I'm going to make some cosmetic changes but the core work here is excellent. Thank you. All the changes in one PR is fine.

[mitchellh]

I know this was still marked as draft but the queueWrite fixes were too good and the remainder looked ready to go to me. If you have some issues let me know! Thank you again, so much!