## 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`.
517a3d0d86