Shard id is not optional

http://distributedlog.io/configuration/proxy.html in the documentation,

server_shard: The shard id used by the server to identify itself. It is optional but recommended to set. For example, if the write proxy is running in Apache Aurora, you could use the instance id as the shard id. The default value is -1 (unset).

However, the shard id is not optional. because the ledger allocator pool uses it to distinguish ledger allocators (see DL-17).

We need to update the documentation to reflect the current state.