notification-thread: remove fd from pollset on LPOLLHUP and friends

notification-thread: remove fd from pollset on LPOLLHUP and friends

When an app dies, it's possible that the notification thread gets an
epoll event (`LPOLLHUP`) that the socket was closed before it gets the
_REMOVE_TRACER_SOURCE command for that source.

In such cases, the notification thread should simply remove the file
descriptor from the pollset and drain the notification on that file
descriptor. It should _not_ remove the _source_element object from the
list.

The removal from the list should only be done when it receives the
_REMOVE_TRACER_SOURCE command.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9525315f9e92d0f6ae5e84e26b83a6b7207dce54