Called when the transport's buffer goes over the high-water mark.
Pause and resume calls are paired -- pause_writing() is called
once when the buffer goes strictly over the high-water mark
(even if subsequent writes increases the buffer size even
more), and eventually resume_writing() is called once when the
buffer size reaches the low-water mark.
Note that if the buffer size equals the high-water mark,
pause_writing() is not called -- it must go strictly over.
Conversely, resume_writing() is called when the buffer size is
equal or lower than the low-water mark. These end conditions
are important to ensure that things go as expected when either
mark is zero.
NOTE: This is the only Protocol callback that is not called
through EventLoop.call_soon() -- if it were, it would have no
effect when it's most needed (when the app keeps writing
without yielding until pause_writing() is called).