Prevent rtx buffer and forwarding path colliding (#1174)

* Prevent rtx buffer and forwarding path colliding

Received packets are put into RTX buffer which is
a circular buffer and the packet (sequence number) is
queued for forwarding. If the RTX buffer fills up
and cycles before forwarding happens, forwarding
would pick the wrong packet (as it is holding a
reference to a byte slice in the RTX buffer) to forward.

Prevent it by moving reading from RTX buffer just
before forwarding. Adds an extra copy from RTX buffer
-> temp buffer for forwarding, but ensures that forwarding
buffer is not used by another go routine.

* Revert some changes from previous commit

Details:
- Do all forward processing as before.
- One difference is not load raw packet into ExtPacket.
- Load raw packet into provided buffer when module that reads
using ReadExtended calls that function. If the packet is
not there in the retransmission buffer, that packet will be
dropped. This is the case we are trying to fix, i. e. the RTX
buffer has cycled before ReadExtended could pull the packet.
This makes a copy into the provided buffer so that the data
does not change underneath.

* Remove debug comment

* Oops missed a function call

pkg/sfu/buffer/buffer_test.go

@@ -146,20 +146,11 @@ func TestNack(t *testing.T) {
 }
 
 func TestNewBuffer(t *testing.T) {
-	type args struct {
-		options Options
-	}
 	tests := []struct {
 		name string
-		args args
 	}{
 		{
 			name: "Must not be nil and add packets in sequence",
-			args: args{
-				options: Options{
-					MaxBitRate: 1e6,
-				},
-			},
 		},
 	}
 	for _, tt := range tests {