[Xrd] Wait for the current PollE poll to finish after removing a Link

[smithdh]

This is a change to avoid issue #1928.

[abh3]

Just comment changes are needed. Presumably the code was tested with the "sleeps" that caused it to fail and now it doesn't fail, right? The only other suggestion is consider repackaging the "eventfd' processing into a separate class so that it can be used elsewhere should the need arise.

[smithdh]

Thanks very much for the comments. I've changed those; the banners and renamed Wait4Poller. I've tried it with my "sleeps" it solves what I could provoke. (I don't know how to reproduce the original production-system problem, as that seems to be quite rare).

Repackaging as a separate class is likely a better way to make it. However I'm not sure I can find a nice general interface for it. (It would probably be used with an epoll-poller, as eventfd is linux specific like epoll). I don't think we have the same problem in XrdSysIOEventsPollE because of how that poller works. So I propose to leave it here (not separated into another class) now.