docs(scheduler): add update docs, from v1.2.2 to v1.2.3, solve handle blob delete msg too slow

Signed-off-by: mawei029 <mawei2@oppo.com>

UPDATE.md

@@ -0,0 +1,87 @@
+# CubeFS
+## blobstore
+- add batch consume kafka msg for scheduler, to solve handle blob delete msg too slow.
+- remove kafka client manage offset local, let kafka server manage offset.
+### scheduler
+** upgrade action **
+
+1.Back up old version
+
+- stop switch
+- check status
+- get current kafka offset
+
+``` shell
+# 1. stop switch, $ip: clustermgr ip, $port: clustermgr port
+curl -XPOST --header 'Content-Type: application/json' http://$ip:$port/config/set -d '{"key":"blob_delete", "value":"false"}'
+curl -XPOST --header 'Content-Type: application/json' http://$ip:$port/config/set -d '{"key":"shard_repair", "value":"false"}'
+
+# 2. check status
+curl http://$ip:$port/config/get?key=blob_delete
+
+# 3. get kafka offset, $ip2: scheduler ip, $port2: scheduler port
+curl $ip2:$port2/metrics | grep kafka_topic_partition_offset | grep consume > metric_offset.log
+```
+
+e.g. metric_offset.log: It has 4 kafka partitions
+> kafka_topic_partition_offset{cluster_id="102",module_name="SCHEDULER",partition="1",topic="blob_delete_102",type="consume"} 1.2053985e+07
+> kafka_topic_partition_offset{cluster_id="102",module_name="SCHEDULER",partition="2",topic="blob_delete_102",type="consume"} 1.2051438e+07
+> kafka_topic_partition_offset{cluster_id="102",module_name="SCHEDULER",partition="3",topic="blob_delete_102",type="consume"} 1234
+> kafka_topic_partition_offset{cluster_id="102",module_name="SCHEDULER",partition="4",topic="blob_delete_102",type="consume"} 1025
+
+2.Set kafka offset
+
+- batch set kafka offset
+
+``` shell
+#!/bin/bash
+
+DIR="/xxx/kafka_2.12-2.3.1-host/bin"  # kafka path
+EXEC="kafka-consumer-groups.sh"
+HOST="192.168.0.3:9095"  # kafka host
+TOPIC="blob_delete_102"
+GROUP="SCHEDULER-$TOPIC"
+array_offset=(12053985  12051438  1234  1025)  # From the file above(metric_offset.log) 
+  
+echo "set consumer offset... ${TOPIC}  ${GROUP}"
+
+cd $DIR
+for i in {0..63}  # number of kafka partitions
+do
+    echo " $i :  ${array_offset[$i]}" 
+    ./$EXEC --bootstrap-server $HOST --reset-offsets --topic ${TOPIC}:$i --group ${GROUP} --to-offset ${array_offset[$i]} --execute
+done
+
+echo "all done..."
+cd -
+
+```
+
+3.update new version
+
+- config file
+
+- stop old version, and install new version
+
+- open switch
+
+``` json
+# ebs-scheduler.conf
+{
+    "blob_delete": {
+        "max_batch_size": 10,  # batch consumption size of kafka messages, default is 10. If the batch is full or the time interval is reached, consume the Kafka messages accumulated during this period
+        "batch_interval_s": 2, # time interval for consuming kafka messages, default is 2s
+        "message_slow_down_time_s": 3,  # slow down when overload, default is 3s
+    }
+}
+
+```
+
+``` shell
+# 1. open switch, $ip: clustermgr ip, $port: clustermgr port
+curl -XPOST --header 'Content-Type: application/json' http://$ip:$port/config/set -d '{"key":"blob_delete", "value":"true"}'
+curl -XPOST --header 'Content-Type: application/json' http://$ip:$port/config/set -d '{"key":"shard_repair", "value":"true"}'
+
+# 2. check status
+curl http://$ip:$port/config/get?key=blob_delete
+```