Change away from using SELECT when open socket

When using SELECT, there is a max of 1024 file descriptor limit. For the Flutter app which each plugin tends to use a lot of file descriptors, it is easy to reach the limit.

From a discussion in a meeting, Jim Borden commented that the reason that we are using SELECT instead of POLL is that the POLL doesn't work on Windows. Also we are using socketpp 3rd library which is using SELECT.