ts/async_wrapper: remove fd from reactor before dropping its handle

The I/O handle was dropped prior to removing it from the reactor, which caused Poller::delete to fail due to an invalid file descriptor. This used to happen silently unless the same fd was added again, e.g. by changing states in the pipeline as follow:

Null -> Playing -> Null -> Playing.

In which case Poller::add failed due to an already existing file.

This commit makes sure the fd is removed from the reactor prior to dropping the handle. In order to achieve this, a new task is spawned on the Context on which the I/O was originally registered, allowing it to access the proper Reactor. The I/O can then safely be dropped.

Because the I/O handle is moved to the spawned future, this solution requires adding the Send + 'static bounds to the I/O handle used within the Async wrapper. This appears not too restrictive for existing implementations though. Other attempts were considered, but they would cause deadlocks.

This new approach also solves a potential race condition where a fd could be re-registered in a Reactor before it was removed.

Fixes: #208 (closed)