Add support for LTR ACK control messages

[ns6089]

Implements #120 proposal.

Notes:

• Code is cleaned up and ready for review.
• Tested on this simple sunshine mock commit and moonlight-qt. LTR ACK messages are being received by sunshine and I didn't break RFI.
• From the performance standpoint it probably makes sense to batch LTR ACK messages together with input so we won't use extra radio time when on wireless. But I didn't trust myself to be able to do this in a non-ugly way. If somebody gets to that in the future, per contract server will be able to handle LTR ACK messages that arrive late and out of order in relation to RFI requests.
• Since it doesn't affect existing functionality and given the release cycle of moonlight-qt, it would be extremely nice to incorporate this into 6.2 release 🙏

[cgutman]

There's a missing initializer warning that's causing CI to fail, but otherwise LGTM.