MINOR: cli: add support for the "show sess all" command

MINOR: cli: add support for the "show sess all" command

Sometimes when debugging haproxy, it is important to take a full
snapshot of all sessions and their respective states. Till now it
was complicated to do because we had to use scripts and sessions
would vanish between two runs.

Now with this command we have the same output as "show sess $id"
but for all sessions in the table. This is a debugging command only,
it should only be used by developers as it is never guaranteed to
perfectly work !