BUG/MINOR: cli/fd: don't isolate the thread for each individual fd

BUG/MINOR: cli/fd: don't isolate the thread for each individual fd

It takes ages to proceed with "show fd" when there is sustained activity
because it uses the rendez-vous point for each and every file descriptor
in the loop. It's very common to see socat timeout there.

Instead of doing this, let's just isolate the function when entering the
loop. Its duration is limited by the number of FDs that may be emitted in
a single buffer anyway, so it's much lighter and responds much faster.