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 commit fixes this closing the connection after receiving all identifiers,
enabling the socket to be cleaned up, which frees up resources.

Apple advises to open the connection on a regular basis and retrieve new
tokens since the last connection. Keeping the connection alive would not
result in getting new identifiers, since Apple closes the connection.