RegionSyncer may lost some data

## Development Task
In the region syncer test: https://ci2.pingcap.net/blue/organizations/jenkins/pd_test/detail/pd_test/3614/pipeline/54/
```
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.907 +08:00] [INFO] [client.go:167] ["server starts to synchronize with leader"] [server=pd2] [leader=pd1] [request-index=0]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.908 +08:00] [INFO] [client.go:167] ["server starts to synchronize with leader"] [server=pd3] [leader=pd1] [request-index=0]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.908 +08:00] [INFO] [server.go:181] ["establish sync region stream"] [requested-server=pd2] [url=http://127.0.0.1:41336]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.908 +08:00] [INFO] [region.go:543] ["leader changed"] [region-id=256] [from=0] [to=0]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.908 +08:00] [INFO] [server.go:181] ["establish sync region stream"] [requested-server=pd3] [url=http://127.0.0.1:34643]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.908 +08:00] [INFO] [server.go:256] ["sync the history regions with server"] [server=pd2] [from-index=0] [last-index=299] [records-length=299]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.909 +08:00] [INFO] [server.go:256] ["sync the history regions with server"] [server=pd3] [from-index=0] [last-index=299] [records-length=299]
[2021-11-24T12:10:23.584Z] [2021/11/24 20:09:15.910 +08:00] [INFO] [region.go:543] ["leader changed"] [region-id=259] [from=0] [to=0]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.913 +08:00] [INFO] [server.go:295] ["start broadcast"] [to-server=pd2] ["start index"=300] [length=1]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.913 +08:00] [INFO] [region.go:543] ["leader changed"] [region-id=262] [from=0] [to=0]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.913 +08:00] [INFO] [server.go:295] ["start broadcast"] [to-server=pd3] ["start index"=300] [length=1]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.916 +08:00] [INFO] [server.go:295] ["start broadcast"] [to-server=pd2] ["start index"=301] [length=1]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.916 +08:00] [INFO] [server.go:295] ["start broadcast"] [to-server=pd3] ["start index"=301] [length=1]
[2021-11-24T12:10:23.585Z] [2021/11/24 20:09:15.917 +08:00] [INFO] [region.go:543] ["leader changed"] [region-id=265] [from=0] [to=0]
```
As the log show, index 299 is lost. which is caused by the sync data and broadcast data have concurrency issue.