Performs actions just before writing the frame to the network.
Some frame, when sent over the network, causes the receiver
to react and send back frames that may be processed by the original
sender *before*
#succeeded() is called.
If the action to perform updates some state, this update may
not be seen by the received frames and cause errors.
For example, suppose the action updates the stream window to a
larger value; the sender sends the frame; the receiver is now entitled
to send back larger data; when the data is received by the original
sender, the action may have not been performed yet, causing the larger
data to be rejected, when it should have been accepted.