Make the Redis broker clean up aborted jobs

The Redis broker uses a non-zero value for `max_retries` to determine
if it should remove a job from the "running" state.  So when an
`AbortException` is thrown, max out the `job.retries` counter,
rather than zeroing out the `job.max_retries` counter.  This will
have the same effect, but play more nicely with what the Redis
broker is expecting.

The MemoryBroker is unaffected by this.

Fixes Issue #20

spinach/job.py

@@ -189,7 +189,7 @@ def advance_job_status(namespace: str, job: Job, duration: float,
         return
 
     if isinstance(err, AbortException):
-        job.max_retries = 0
+        job.retries = job.max_retries
         logger.error(
             'Fatal error during execution of %s after %s, canceling retries',
             job, duration, exc_info=err

tests/test_job.py

@@ -118,10 +118,13 @@ def test_advance_job_status(job):
                        RetryException('Must retry', at=now))
     assert job.status is JobStatus.FAILED
 
+    # The job should have retried twice due to previous tests, ensure that
+    # an AbortException tops off the retry counter
     job.status = JobStatus.RUNNING
     job.max_retries = 10
+    assert job.retries == 2
     advance_job_status('namespace', job, 1.0, AbortException('kaboom'))
-    assert job.max_retries == 0
+    assert job.retries == 10
     assert job.status is JobStatus.FAILED