use callbacks for handling results from worker processes.

Previously, lot of code in the workers module was about handling the output of the commands in a job-type specific way. For example, parts of check result handling was implemented in workers.c, which actually should be in checks.c.

This pull request changes all wproc_run_*-calls to wproc_run-callback, so the result handling and domain specific logging is done in the file related to the handler, so wproc becomes a simple generic api for executing commands thorugh workers.

Reduced dependencies makes it easier to mock up and implement tests.