Add spawn worker-type to one server configurations block

[daipom]

These configs can be used in "spawn" worker-type too.

- Multithread server and multiprocess server: available only when `worker_type` is thread or process
  - **workers** sets number of workers (default: 1) [dynamic reloadable]
  - **start_worker_delay** sets the delay between each worker-start when starting/restarting multiple workers at once (default: 0) [dynamic reloadable]
  - **start_worker_delay_rand** randomizes start_worker_delay at this ratio (default: 0.2) [dynamic reloadable]
  - **restart_worker_interval** sets wait time before restarting a stopped worker (default: 0) [dynamic reloadable]

[ashie]

:+1:

[daipom]

Thanks for the merge!