Tune worker/job counts (#5346)

* デフォルトのワーカー数を1に

* Tune default job count

.config/example.yml

@@ -116,8 +116,12 @@ autoAdmin: true
 # Whether disable HSTS
 #disableHsts: true
 
-# Clustering
+# Number of worker processes
 #clusterLimit: 1
 
+# Job concurrency per worker
+# deliverJobConcurrency: 128;
+# inboxJobConcurrency: 16;
+
 # IP address family used for outgoing request (ipv4, ipv6 or dual)
 #outgoingAddressFamily: ipv4