kv: iterator support

[barraguda]

Problem

hyperware-ai/process_lib#125

Solution

pub enum KvAction {
    // ...
    IterStart { prefix: Option<Vec<u8>> },
    IterNext { iterator_id: u64, count: u64 },
    IterClose { iterator_id: u64 },
}

pub enum KvResponse {
    // ...
    IterStart { iterator_id: u64 },
    IterNext { done: bool },
    IterClose { iterator_id: u64 },
}

Testing

# Follow these steps to test

[nick1udwig]

Seems reasonable to me. I'd recommend closing the iterator once you hit done = true rather than requiring the consumer to call IterClose.

In general DashMaps make me nervous cuz we've had a lot of deadlocks from them, but just looking at the code it seems ok 😅

Please be sure to hit all the new codepaths here in tests, ideally with two processes to try to exercise the DashMaps in a multi-tokio::task context.

[dr-frmr]

Comment nits, please try to give interface that final polish.

Echoing Nick's concern here about lockups when multiple processes are iterating on same DB, please make sure to test that.

[dr-frmr]

Comments inside enums should be /// docstrings -- please label each iter operation with an explanation of how to use it. It's not immediately obvious to me how it works.