qa: update the rbd/concurrent.sh workunit

A few changes, now that a few rbd problems have been fixed.
    - Don't run with "set -x".  It produces a ton of noise that is
      not useful for this test.  This is an exerciser, looking
      really for system crashes during concurrent activity, and
      knowing which commands were (concurrently) active isn't going
      to help much in diagnosis.
    - Create two more directories, used to track the degree of
      concurrency (more or less) and the highest rbd id consumed.
      Files whose names are numbers are touched in each, and the
      highest at the end is the highest during the run.  This gets
      around issues passing environment info from sub-shells to the
      top-level shell.  As a bonus, it offers a better chance of
      avoiding problems due to concurrent update.
    - NAMESDIR is renamed NAMES_DIR, and it (and the others) is
      set up in the setup() function.
    - Increase the concurrency and iteration counts.

Signed-off-by: Alex Elder <elder@inktank.com>