Changing concurrency dynamically

As I understand there is no easy way now to change concurrency on the fly.

Starting more processor could work to increase concurrency:

``` js
jobs.process('email', 10, function processEmail(job, done){...});
// later, add 5 to concurrency
jobs.process('email', 5, processEmail);
```

Looks hacky, and how do I lower concurrency?

The issue names jobs.process as the entry point; start by tracing how its concurrency argument is applied to a running worker. Clarify the expected behavior for increasing and decreasing concurrency, then define tests for both directions; done means concurrency can be changed dynamically without restarting the worker.

Written by the indexing model from the issue text.