MINOR: stream/cli: add an optional "older" filter for "show sess"

MINOR: stream/cli: add an optional "older" filter for "show sess"

It's often needed to be able to refine "show sess" when debugging, and
very often a first glance at old streams is performed, but that's a
difficult task in large dumps, and it takes lots of resources to dump
everything.

This commit adds "older <age>" to "show sess" in order to specify the
minimum age of streams that will be dumped. This should simplify the
identification of blocked ones.