[proxy]: BatchQueue::call is not cancel safe - make it directly cancellation aware (#12345)

## Problem

https://github.com/neondatabase/cloud/issues/30539

If the current leader cancels the `call` function, then it has removed
the jobs from the queue, but will never finish sending the responses.
Because of this, it is not cancellation safe.

## Summary of changes

Document these functions as not cancellation safe. Move cancellation of
the queued jobs into the queue itself.

## Alternatives considered

1. We could spawn the task that runs the batch, since that won't get
cancelled.
* This requires `fn call(self: Arc<Self>)` or `fn call(&'static self)`.
2. We could add another scopeguard and return the requests back to the
queue.
* This requires that requests are always retry safe, and also requires
requests to be `Clone`.

proxy/src/redis/keys.rs

@@ -23,9 +23,8 @@ impl KeyPrefix {
 
 #[cfg(test)]
 mod tests {
-    use crate::pqproto::id_to_cancel_key;
-
     use super::*;
+    use crate::pqproto::id_to_cancel_key;
 
     #[test]
     fn test_build_redis_key() {