MEDIUM: polling: start to move maxfd computation to the pollers

MEDIUM: polling: start to move maxfd computation to the pollers

Since only select() and poll() still make use of maxfd, let's move
its computation right there in the pollers themselves, and only
during each fd update pass. The computation doesn't need a lock
anymore, only a few atomic ops. It will be accurate, be done much
less often and will not be required anymore in the FD's fast patch.

This provides a small performance increase of about 1% in connection
rate when using epoll since we get rid of this computation which was
performed under a lock.