MINOR: cli: make "show sess" stop at the last known session

MINOR: cli: make "show sess" stop at the last known session

"show sess" and particularly "show sess all" can be very slow when dumping
lots of information, and while dumping, new sessions might appear, making
the output really endless. When threads are used, this causes a double
problem:
  - all threads are paused during the dump, so an overly long dump degrades
    the quality of service ;

  - since all threads are paused, more events get postponed, possibly
    resulting in more streams to be dumped on next invocation of the dump
    function.

This patch addresses this long-lasting issue by doing something simple:
the CLI's stream is moved at the end of the steams list, serving as an
identifiable marker to end the dump, because all entries past it were
added after the command was entered. As a result, the CLI's stream always
appears as the last one.

It may make sense to backport this to stable branches where dumping live
streams is difficult as well.