Hold lock when accessing remote description
Hold lock when accessing remote description

CurrentRemoteDescription and PendingRemoteDescription both access
members of the PeerConnection that should be protected by a lock.

Co-authored-by: Markus Tzoe <4d36932e3b01fa53f3a6e5be5b5d6bb976119e05@gmail.com>

pion/webrtc,pion/webrtc,pion/webrtc,pion/webrtc