May sync unit wait relay unit initialized

[csuzhangxc]

Feature Request

Is your feature request related to a problem? Please describe:

sync unit may be reading binlog events which may be recovered/truncated by relay unit.

DM v1.0.6 log:

[2020/10/21 17:54:38.421 +08:00] [INFO] [syncer.go:1093] ["replicate binlog from checkpoint"] [task=sdapi_increment_worker2] [unit="binlog replication"] [checkpoint="(binlog|000001.001786, 734976074)"]

[2020/10/21 17:54:38.436 +08:00] [INFO] [relay.go:340] ["started recover writer"] [component="relay log"] [UUID=e7299461-ff7e-11e9-8021-d09466509bd9.000001] [config="{\"RelayDir\":\"/data/dm-work2/relay_log/e7299461-ff7e-11e9-8021-d09466509bd9.000001\",\"Filename\":\"binlog.001786\"}"]

[2020/10/21 17:59:44.343 +08:00] [WARN] [relay.go:345] ["relay log file recovered"] [component="relay log"] ["from position"="(binlog.001786, 735007222)"] ["to position"="(binlog.001786, 735000050)"] ["from GTID set"=] ["to GTID 

[2020/10/21 17:59:44.423 +08:00] [ERROR] [file.go:184] ["size of relay log file has been changed"] [file=/data/dm-work2/relay_log/e7299461-ff7e-11e9-8021-d09466509bd9.000001/binlog.001786] ["old size"=735007222] [size=735000115] 

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: