Close APN::Feedback connection after receiving tokens

After retrieving all invalid device identifiers from Apple via `APN::Feedback#get`, Apple closes the connection and the socket stays opened in a `CLOSE_WAIT` state and gets not cleaned up and closed.

This PR fixes this by closing the connection after receiving all identifiers, enabling the socket to be cleaned up, which frees up resources.

For more information, see the [Apple Documentation about their Feedback Service](http://developer.apple.com/library/mac/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/CommunicatingWIthAPS.html#//apple_ref/doc/uid/TP40008194-CH101-SW3)

The documentation does not describe that opening a connection to the Feedback Service on a regular basis will result in a ban. It rather says the opposite: you should reopen the connection on a regular basis to retrieve new tokens.

And since Apple closes the connection, I think it's a good idea to close the connection too, to free up resources.

Thoughts?