Inserts the new frame at the correct location in the pending frame list. Note that this must
never insert a frame at the very start of the senders list, as this frame has already been activated.
This method should return true if the frame was successfully inserted into the pending frame list,
if it returns false the frame must not be inserted and will be added to the held frames list instead.
Frames held in the held frames list are frames that are not yet ready to be included in the pending frame
list, generally because other frames have to be written first.
Note that if this method returns true without adding the frame the frame will be dropped.
newFrame - The new frame to insert into the pending frame list
pendingFrames - The pending frame list
true if the frame can be inserted into the pending frame list
Invoked when a new frame is successfully added to the pending frames queue.
If frames in the held frame queue are now eligible to be sent they can be added
to the pending frames queue.
Note that if the protocol has explicitly asked for the held frames to be recalculated
then the added frame may be null.