Save prev commit seq if progress is done (selectdb#91)

Fix the bug introduced by selectdb#86.

The prev commit seq is indicates the commit seq where the target
cluster has synced, so once restoring is finished, the prev commit
seq should be set to aligned with commit seq

pkg/ccr/job_progress.go

@@ -136,6 +136,7 @@ type JobProgress struct {
 	// Sub sync state machine states
 	SubSyncState SubSyncState `json:"sub_sync_state"`
 
+	// The commit seq where the target cluster has synced.
 	PrevCommitSeq     int64           `json:"prev_commit_seq"`
 	CommitSeq         int64           `json:"commit_seq"`
 	TableMapping      map[int64]int64 `json:"table_mapping"`
@@ -264,6 +265,9 @@ func (j *JobProgress) CommitNextSubWithPersist(commitSeq int64, subSyncState Sub
 	j.Persist()
 }
 
+// Switch to new sync state.
+//
+// The PrevCommitSeq is set to commitSeq, if the sub sync state is done.
 func (j *JobProgress) NextWithPersist(commitSeq int64, syncState SyncState, subSyncState SubSyncState, persistData string) {
 	if subSyncState == BeginCreateSnapshot && (syncState == TableFullSync || syncState == DBFullSync) {
 		j.FullSyncStartAt = time.Now().Unix()
@@ -275,6 +279,10 @@ func (j *JobProgress) NextWithPersist(commitSeq int64, syncState SyncState, subS
 	}
 
 	j.CommitSeq = commitSeq
+	if subSyncState == Done {
+		j.PrevCommitSeq = commitSeq
+	}
+
 	j.SyncState = syncState
 	j.SubSyncState = subSyncState
 	j.PersistData = persistData