fix: Fix Goroutine leak after calling Close

[gabe565]

Hello!

go-chromecast is an amazing tool. I am writing a Go implementation of sponsorblockcast called CastSponsorSkip and have noticed if a device disconnects and I call Application.Close(), some Goroutines never exit resulting in a slight memory leak.

The problematic functions are:

• Application.messageChanHandler() due to Application.messageChan not being closed.
• Application.recvMessages() due to Connection.recvMsgChan not being closed.

This PR closes those channels when Application.Close() and Connection.Close() are called. I opted to close the channels in a defer so that they stay open while the underlying connection is closed. Let me know if you have any feedback!

[vishen]

Ah, thanks @gabe565, good catch! PR looks good :)