AIX: (ch) Fix synchronization issue in AixPollPort.java

On AIX we use the AIXPollPort class to wrap the native pollset functionality.

AIXPollPort is based on the Linux EPollPort class. But on AIX there's an extra difficulty - we can not modify (with pollset_ctl()) and query (with pollset_poll()) a pollset concurrently (like this is possible on Linux with epoll_ctl() and epoll_wait()).

We have therefore synchronized calls to pollset_ctl() and pollset_poll() by using an extra control queue which handles these calls sequentially.

This implementation has an error which can lead to a deadlock if we have a pending pollset_ctl() event in the control queue while the corresponding pollset is already empty.