Introduce concurrency_blocking and use it in Node.apply_items
BlockingWorkerPool: Refine API

- Handle jobs_open completely internally
- Handle LOG internally
- Re-raise exceptions in the parent
- Simplify msg['result']
Use BlockingWorkerPool in run_actions
Use BlockingWorkerPool in "bw apply"
Use BlockingWorkerPool in "bw run"
Use BlockingWorkerPool in "bw verify"
Remove old concurrency module
New concurrency.py: Revise comments
Remove old concurrency tests

Coverage drops, of course, but these test cases are simply incompatible
with the new module.
Merge branch 'refs/heads/conc-rewrite'