Include setproctitle, so celery processes are easier to distinguish.

Instead of all of them being called `celery`, they now distinguish
between the beat, worker main process and individual worker processes.
Pin down importlib, which gets pulled in as a dependency under Python 2.6.
Revised sketch for issues #205, #206 and part of #153.
Add more metrics, tests for search-cross-checks. Close #205, close #206.