Really fiber-aware ActiveRecord connection pool and em_mysql2 adapter

This implementation supports concurrent running transactions. We tested it with active_record 3.2, but did not test with 4.0. We are using it in production and it is working well.

ActiveRecord have already had good connection pooling, that uses Thread.current[] for thread-aware (and fiber-aware) storage. We replaced original MonitorMixin, by fiber-aware one, and thats all.