Introduce ThreadedConnectionPool for customized pool

This way we could reuse this pool for other migrations

Feedback:

* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8987#note_22923350
* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8987#note_22923365

db/post_migrate/20170206040400_remove_inactive_default_email_services.rb

@@ -3,38 +3,27 @@ class RemoveInactiveDefaultEmailServices < ActiveRecord::Migration
 
   DOWNTIME = false
 
+  disable_ddl_transaction!
+
   def up
-    builds_service = spawn <<-SQL.strip_heredoc
+    Gitlab::Database::ThreadedConnectionPool.with_pool(2) do |pool|
+      pool.execute_async <<-SQL.strip_heredoc
         DELETE FROM services
           WHERE type = 'BuildsEmailService'
             AND active IS FALSE
             AND properties = '{"notify_only_broken_builds":true}';
       SQL
 
-    pipelines_service = spawn <<-SQL.strip_heredoc
+      pool.execute_async <<-SQL.strip_heredoc
         DELETE FROM services
           WHERE type = 'PipelinesEmailService'
             AND active IS FALSE
             AND properties = '{"notify_only_broken_pipelines":true}';
       SQL
-
-    [builds_service, pipelines_service].each(&:join)
-  end
-
-  private
-
-  def spawn(query)
-    Thread.new do
-      with_connection do |connection|
-        connection.execute(query)
-      end
     end
   end
 
-  def with_connection(&block)
-    pool = ActiveRecord::Base.establish_connection
-    pool.with_connection(&block)
-  ensure
-    pool.disconnect!
+  def down
+    # Nothing can be done to restore the records
   end
 end

lib/gitlab/database/threaded_connection_pool.rb

+module Gitlab
+  module Database
+    class ThreadedConnectionPool
+      def self.with_pool(pool_size)
+        pool = new(pool_size)
+
+        yield(pool)
+
+      ensure
+        pool.join
+        pool.close
+      end
+
+      def initialize(pool_size)
+        config = ActiveRecord::Base.configurations[Rails.env]
+        @ar_pool = ActiveRecord::Base.establish_connection(
+          config.merge(pool: pool_size))
+        @workers = []
+        @mutex = Mutex.new
+      end
+
+      def execute_async(sql)
+        @mutex.synchronize do
+          @workers << Thread.new do
+            @ar_pool.with_connection do |connection|
+              connection.execute(sql)
+            end
+          end
+        end
+      end
+
+      def join
+        threads = nil
+
+        @mutex.synchronize do
+          threads = @workers.dup
+          @workers.clear
+        end
+
+        threads.each(&:join)
+      end
+
+      def close
+        @ar_pool.disconnect!
+      end
+    end
+  end
+end