core: protect against changes to the pollfd list during handle_events

It is possible a file descriptor to be removed due to a close and the
same file descriptor value to be added during the same event loop. This
can cause the backend to get a stale revent and incorrectly remove the
file descriptor.

This commit addresses the issue by delaying the actual removal of the
ipollfds entry until just before the backend handle_events is called.
handle_events then goes through the list of closed fds and clears out
the revents associated with the closed file descriptor.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>