[WIP] connect_timeout option for TCPSocket.new

I met troubles that fluentd does not terminate sometimes. 
Using [sigdump](https://github.com/frsyuki/sigdump), I found that fluentd was stuck at `TCPSocket.new` on shutdown. Please see https://gist.github.com/sonots/e05d8fbcd7ab39e4a5f8 for details. 

So, I implemented `SocketUtil.create_tcp_socket` and `SocketUtil.open_tcp_socket` having `connect_timeout` option as https://bugs.ruby-lang.org/issues/5101 says. 

PS. Please wait to merge. I will test for a week on my production environment.