More precise ack batch calculation for subscriber

We assume some conservative upper limit on the number of ack IDs that fit in an `AcknowledgeRequest` and `ModifyAckDeadlineRequest`. It seems that ack IDs are of variable length[^1]. As @feywind points out, it would be more precise to measure the size of the protobuf request as we fill it, and use that to determine when a batch is full.

We would probably modify our `Leases` class to keep track of the `to_ack` / `to_nack` size on `Leases::add()`. Gemini tells me we should add `3 + ack_id.len()`, because the protobuf encoding for a string takes 3 bytes. We could cut off the request size at like 500 kB, and probably fill more ack IDs in a batch than we are doing now.

This might have a slight performance bump for very high throughput applications.

[^1]: maybe a function of the subscription length?