MEDIUM: pollers: Drop fd events after a takeover to another tgid.

MEDIUM: pollers: Drop fd events after a takeover to another tgid.

In pollers that support it, provide the generation number in addition to
the fd, and, when an event happened, if the generation number is the
same, but the tgid changed, then assumed the fd was taken over by a
thread from another thread group, and just delete the event from the
current thread's poller, as we no longer want to hear about it.