refactor NIDAS reactor code

[garyjg]

Currently there are several almost identical sections of code which all implement a select loop. Either consolidate and encapsulate all that code into a reactor API, or use a third-party implementation like boost::asio, and add support for timer events. Then use the timer events to properly implement timeouts which do not depend on other events to trigger the timeout check. In the current implementation, all the CPP conditional compile code can be cleaned up, since all platforms support epoll() now.