Try small payload chunks in GET

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when a single GRPC message spans multiple HTTP/2 frames. These frames need to be queued before returning them to GRPC stream handler. With 254KB of data per message that's 16 frames.

Describe the solution you'd like

We still want to read ~256KB from disk, but we can try sending this data in smaller messages. Each message needs to be ~15KB to perfectly fit into the default HTTP/2 frame. Then we can check how this affects local reads and forwarding.

Likely this will require some buffer dances with grpc prefixes stored in a different slice from payload (currently a single one is used).

[roman-khimov]

Tried in #4194, resulted in 20-30%% throughput decrease, not needed.