Fixes a deadlock because a WaitGroup didn't decrement
Fixes a deadlock because a WaitGroup didn't decrement

On the one hand, we were limiting the number of active
SingularityRequest capturing goroutines to the number of unique
Singularity URLs.

BUT, we were counting all the clusters and waiting for them all to
finish - so if there were two "clusters" with the same URL, (i.e.
because different deployment enviroments (ci vs. pp), one would get
skipped and never be marked `Done().`

Also handled: the stray "cannot retry: <nil>" message that popped up
because the error channel closed to indicate completion.

Finally, some extra debug logging.

samsalisbury/sous,opentable/sous,samsalisbury/sous,opentable/sous,samsalisbury/sous,nyarly/sous,nyarly/sous,opentable/sous,samsalisbury/sous,opentable/sous,samsalisbury/sous,nyarly/sous,opentable/sous