Move async execution from celluloid to concurrent-ruby

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```

2 files changed, 44 insertions, 16 deletions

diff --git a/actioncable/lib/action_cable/server/base.rb b/actioncable/lib/action_cable/server/base.rb
index 740e4b301e..cfd0a65f0f 100644
--- a/actioncable/lib/action_cable/server/base.rb
+++ b/actioncable/lib/action_cable/server/base.rb
@@ -1,6 +1,3 @@
-# FIXME: Cargo culted fix from https://github.com/celluloid/celluloid-pool/issues/10
-require 'celluloid/current'
-
 require 'em-hiredis'
 
 module ActionCable
diff --git a/actioncable/lib/action_cable/server/worker.rb b/actioncable/lib/action_cable/server/worker.rb
index e063b2a2e1..6cddb4e7a5 100644
--- a/actioncable/lib/action_cable/server/worker.rb
+++ b/actioncable/lib/action_cable/server/worker.rb
@@ -1,39 +1,70 @@
-require 'celluloid'
 require 'active_support/callbacks'
+require 'concurrent'
 
 module ActionCable
   module Server
     # Worker used by Server.send_async to do connection work in threads. Only for internal use.
     class Worker
       include ActiveSupport::Callbacks
-      include Celluloid
 
-      attr_reader :connection
       define_callbacks :work
       include ActiveRecordConnectionManagement
 
+      def initialize(max_size=5)
+        @pool = Concurrent::ThreadPoolExecutor.new(
+          min_threads: 1,
+          max_threads: max_size,
+          max_queue: 0,
+        )
+      end
+
+      def connection
+        Thread.current[:connection] || raise("No connection set")
+      end
+
+      def async_invoke(receiver, method, *args)
+        @pool.post do
+          invoke(receiver, method, *args)
+        end
+      end
+
       def invoke(receiver, method, *args)
-        @connection = receiver
+        begin
+          Thread.current[:connection] = receiver
+
+          run_callbacks :work do
+            receiver.send method, *args
+          end
+        rescue Exception => e
+          logger.error "There was an exception - #{e.class}(#{e.message})"
+          logger.error e.backtrace.join("\n")
 
-        run_callbacks :work do
-          receiver.send method, *args
+          receiver.handle_exception if receiver.respond_to?(:handle_exception)
+        ensure
+          Thread.current[:connection] = nil
         end
-      rescue Exception => e
-        logger.error "There was an exception - #{e.class}(#{e.message})"
-        logger.error e.backtrace.join("\n")
+      end
 
-        receiver.handle_exception if receiver.respond_to?(:handle_exception)
+      def async_run_periodic_timer(channel, callback)
+        @pool.post do
+          run_periodic_timer(channel, callback)
+        end
       end
 
       def run_periodic_timer(channel, callback)
-        @connection = channel.connection
+        begin
+          Thread.current[:connection] = channel.connection
 
-        run_callbacks :work do
-          callback.respond_to?(:call) ? channel.instance_exec(&callback) : channel.send(callback)
+          run_callbacks :work do
+            callback.respond_to?(:call) ? channel.instance_exec(&callback) : channel.send(callback)
+          end
+        ensure
+          Thread.current[:connection] = nil
         end
       end
 
       private
+
         def logger
           ActionCable.server.logger
         end