More consistent -[close] behavior

This means refusing to close twice, calling -[close] from -[dealloc] and
not calling -[cancelAsyncRequests].

Calling -[cancelAsyncRequests] in -[close] is too dangerous, as -[close]
gets called by -[dealloc]: If the queue is the last reference to the
object, at the point where -[cancelAsyncRequests] removes it from the
queue, the object will start to deallocate and call into
-[cancelAsyncRequests] again, which is still in the middle of removing
it and now finds itself with an inconsistent state.