Make RTCRtpTransceiver.mid spec-compliant

[rogurotus]

The current implementation is:

pub struct RTCRtpTransceiver {
    mid: Mutex<String>,                           //atomic.Value
    sender: Mutex<Option<Arc<RTCRtpSender>>>,     //atomic.Value
    receiver: Mutex<Option<Arc<RTCRtpReceiver>>>, //atomic.Value
    ...
}

Essentially, the empty string is used instead of None (Option<String>).
This PR makes the behavior of mid explicitly optional, and also replaces Mutex with OnceCell.

[rogurotus]

ping @k0nserv

[codecov]

Codecov Report

Base: 60.14% // Head: 61.12% // Increases project coverage by +0.98% 🎉

Coverage data is based on head (8d2edb9) compared to base (6506f2c).
Patch coverage: 48.14% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   60.14%   61.12%   +0.98%     
==========================================
  Files         503      537      +34     
  Lines       47987    49620    +1633     
  Branches    12501    12590      +89     
==========================================
+ Hits        28860    30330    +1470     
- Misses       9911     9968      +57     
- Partials     9216     9322     +106     

Impacted Files	Coverage Δ
webrtc/src/rtp_transceiver/rtp_transceiver_test.rs	53.27% <0.00%> (+0.46%)	⬆️
...tc/src/peer_connection/peer_connection_internal.rs	57.43% <50.00%> (+0.54%)	⬆️
sctp/src/queue/pending_queue.rs	33.33% <0.00%> (-55.24%)	⬇️
sctp/src/queue/payload_queue.rs	76.31% <0.00%> (-9.40%)	⬇️
sctp/src/queue/queue_test.rs	65.12% <0.00%> (-3.59%)	⬇️
webrtc/src/mux/mux_func.rs	75.00% <0.00%> (-3.58%)	⬇️
rtcp/src/header.rs	63.93% <0.00%> (-3.28%)	⬇️
sdp/src/lexer/mod.rs	66.66% <0.00%> (-3.04%)	⬇️
data/src/error.rs	17.94% <0.00%> (-2.64%)	⬇️
data/src/message/message_channel_open.rs	79.51% <0.00%> (-2.41%)	⬇️
... and 100 more

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[k0nserv]

Excellent change 🎉

Just a few comments

[k0nserv]

@rogurotus I'm sorry, I'm a doofus, we can't use let-else as I suggested because of our MSRV policy 🤦🏼

Instead of it can you use match as

let x = match y {
   //Positive match yields value,
   _ => continue; // or break/return etc
}

[k0nserv]

LGTM