Add (best-effort) stealing API to polling system

[armanbilge]

In #3873 (comment) we decided that we will support stealing for load-balancing. Thus, we should add an API to the polling system that allows completed I/O to be stolen. Not all polling systems will be able to implement this, but hopefully some of them can, such as io_uring, see #3873 (comment).

[Dralt03]

I wanted to work on this issue.
Also I was hoping you could tell me how you want to proceed forward with this, I saw your PR and I was just wondering why it hadn't been merged yet

[armanbilge]

I wanted to work on this issue. Also I was hoping you could tell me how you want to proceed forward with this, I saw your PR and I was just wondering why it hadn't been merged yet

@Dralt03 The reason it isn't merged yet is explained in #4113 (review). Basically we want to have a concrete usage / implementation before merging.