Meta server: change chunk server and chunk access response format to make chunk server access optional. Emit chunk server access tokens with write lease renew only if chunk server explicitly request them. Add chunk server access "valid for time" to the chunk allocation request and lease renew response to avoid parsing of the chunk server access tokens.
Chunk server: change chunk server and chunk access related logic to keep the two separate with write lease. Explicitly request to update chunk access tokens with write lease renew, before chunk server access tokens expire. Change chunk server access and chunk access forwarding logic to work with the new chunk access and chunk server access format.