Add a new option to set delay to restart a stopped worker

This is still in POC stage.

This tries to solve the problem that restarting each worker is delayed
one by one, with as simple as possible.

However, this fix can't prevent the above problem...

lib/serverengine/multi_worker_server.rb

@@ -85,6 +85,7 @@ def reload_config
 
       @start_worker_delay = @config[:start_worker_delay] || 0
       @start_worker_delay_rand = @config[:start_worker_delay_rand] || 0.2
+      @restart_worker_delay = @config[:restart_worker_delay] || 0
 
       scale_workers(@config[:workers] || 1)
 
@@ -97,6 +98,7 @@ def wait_tick
 
     def keepalive_workers
       num_alive = 0
+      is_restarting = false
 
       @monitors.each_with_index do |m,wid|
         if m && m.alive?
@@ -116,6 +118,10 @@ def keepalive_workers
         elsif wid < @num_workers
           # scale up or reboot
           unless @stop
+            if m && @restart_worker_delay > 0 && !is_restarting
+              is_restarting = true
+              sleep @restart_worker_delay
+            end
             @monitors[wid] = delayed_start_worker(wid)
             num_alive += 1
           end