MAJOR: cli: Update the CLI applet to handle its own buffers

MAJOR: cli: Update the CLI applet to handle its own buffers

It is the third applet to be refactored to use its own buffers. In addition to
the CLI applet, some I/O handlers of CLI commands were also updated, especially
the stats ones.

Some command I/O handlers were updated to use applet's buffers instead of
channels ones.