Shut down the worker pool - don't kill it

Different parts of concurrent-ruby's documentation make inconsistent
claims about how kill will behave. It doesn't do the thing we want.

actioncable/lib/action_cable/server/worker.rb

@@ -25,7 +25,7 @@ def initialize(max_size: 5)
       # Stop processing work: any work that has not already started
       # running will be discarded from the queue
       def halt
-        @executor.kill
+        @executor.shutdown
       end
 
       def stopping?