MINOR: stats: Use a dedicated function to check if output is almost full

MINOR: stats: Use a dedicated function to check if output is almost full

This simplifies a bit the stats applet. Because the CLI part was not
refactored yet to use the applet's buffers, there are 3 ways to produce
data:

  * the HTX message for the HTTP stats when zero-copy forwarding is not
    used
  * raw data in the opposite endpoint buffer for the HTTP stats when
    zero-copy forwarding is used
  * the channel buffer when the CLI "show stat" command is evaluated

There is already a dedicated function to take care to copy data at the right
place. There is now also a dedicated function to check us the output buffer
is almost full.