Reduce Parsoid job processing parallelism further

Brandon fixed timeout issues in only-if-modified requests, which increases the
throughput of Parsoid template update jobs. This patch reduces the job
parallelism to keep parsoid job processing throttled enough to avoid
overloading the API cluster, which currently has very little spare capacity.

Change-Id: I72a88ae3904328e1095b6756077e91324487c43e