qa: update the rbd/concurrent.sh workunit

A few changes, now that a few rbd problems have been fixed.
First, the more substantive changes:
    - Generate a source file, and compare what's read back from rbd
      devices with the content of that file.
    - Write to the rbd device such that the written data spans
      an (assumed 4 MB) rbd object boundary, as well as starting
      and ending on non-page-aligned offsets.
    - Perform multiple reads on rbd devices: entirely within a range
      before any written data; beginning before but ending within
      written data; the exact written data (and validating what's
      read); beginning within written data but ending after it;
      reading after written data but within a written rbd object;
      and reading from an unwritten rbd object.
    - Have the sleep between iterations provide a non-integer value
      to avoid zero (or quantized) delays.

Also, some a little less substantive (but possibly informative):
    - 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>