Chunk server: increase client thread stack size to 256KB, as 32KB might not be sufficient to handle deep recursion.
Chunk server: allow to configure command header read ahead size. Reducing the size to an average rpc header size should increase fairness and decrease number of header parsed with the client threads mutex acquired, possibly at the coast of additional system calls. It is possible to avoid additional system calls at the cost of complexity increase in the client threads re-scheduling logic, as an extra state, or dispatch / "drain" loop would be required.