MAJOR: cli: Refacor parsing and execution of pipelined commands

MAJOR: cli: Refacor parsing and execution of pipelined commands

Before this patch, when pipelined commands were received, each command was
parsed and then excuted before moving to the next command. Pending commands
were not copied in the input buffer of the applet. The major issue with this
way to handle commands is the impossibility to consume inputs from commands
with an I/O handler, like "show events" for instance. It was working thanks
to a "bug" if such commands were the last one on the command line. But it
was impossible to use them followed by another command. And this prevents us
to implement any streaming support for CLI commands.

So we decided to refactor the command line parsing to have something similar
to a basic shell. Now an entire line is parsed, including the payload,
before starting commands execution. The command line is copied in a
dedicated buffer. "appctx->chunk" buffer is used for this purpose. It was an
unsed field, so it is safe to use it here. Once the command line copied, the
commands found on this line are executed. Because the applet input buffer
was flushed, any input can be safely consumed by the CLI applet and is
available for the command I/O handler. Thanks to this change, "show event
-w" command can be followed by a command. And in theory, it should be
possible to implement commands supporting input data streaming. For
instance, the Tetris like lua applet can be used on the CLI now.

Note that the payload, if any, is part of the command line and must be fully
received before starting the commands processing. It means there is still
the limitation to a buffer, but not only for the payload but for the whole
command line. The payload is still necessarily at the end of the command
line and is passed as argument to the last command. Internally, the
"appctx->cli_payload" field was introduced to point on the payload in the
command line buffer.

This patch is quite huge but it cannot easily be splitted. It should not
introduced significant changes.