seata-server.log __ rm client is not connected.

- [ ] I have searched the [issues](https://github.com/seata/seata/issues) of this repository and believe that this is not a duplicate.

### Ⅰ. Issue Description

seata-server 线上环境日志经常打印此异常

2020-07-29 17:16:25.917 ERROR[AsyncCommitting_1]io.seata.server.coordinator.DefaultCore.doGlobalCommit:218 -Exception committing branch BR:2041145439/2041145433
java.lang.RuntimeException: rm client is not connected. dbkey:jdbc:mysql://xxx3:3306/xxx,clientId:demand-service:192.168.65.xx:43244
at io.seata.core.rpc.netty.RpcServer.sendSyncRequest(RpcServer.java:112)
at io.seata.core.rpc.netty.RpcServer.sendSyncRequest(RpcServer.java:164)
at io.seata.server.coordinator.AbstractCore.branchCommitSend(AbstractCore.java:164)
at io.seata.server.coordinator.AbstractCore.branchCommit(AbstractCore.java:154)
at io.seata.server.coordinator.DefaultCore.doGlobalCommit(DefaultCore.java:186)
at io.seata.server.coordinator.DefaultCoordinator.handleAsyncCommitting(DefaultCoordinator.java:345)
at io.seata.server.coordinator.DefaultCoordinator.lambda$init$3(DefaultCoordinator.java:399)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.lang.Thread.run(Thread.java:748)

### Ⅱ. Describe what happened

暂不详知

### Ⅲ. Describe what you expected to happen

找到发生此异常的原因，避免下次出现，解决问题

### Ⅳ. How to reproduce it (as minimally and precisely as possible)

业务系统线上问题，暂无复现场景

### Ⅴ. Anything else we need to know?

无

### Ⅵ. Environment:

- JDK version : 1.8
- OS : 2.6.32-431.el6.x86_64
- Others: seata-server 1.2.0

Start with the stack-trace entry points io.seata.core.rpc.netty.RpcServer.sendSyncRequest and io.seata.server.coordinator.AbstractCore.branchCommitSend, then inspect how the RM client connection is handled during branch commit. A reproducible case is still needed; done means identifying the cause of the disconnected client error and preventing or correctly handling the recurring production failure.

Written by the indexing model from the issue text.