[3.1.6 Backport] CBG-3845: Ensure sequence update waits for callback …

…invocation (#6797)

Backports #6773 to 3.1.6

base/dcp_common.go

@@ -120,8 +120,8 @@ func NewDCPCommon(ctx context.Context, callback sgbucket.FeedEventCallbackFunc,
 }
 
 func (c *DCPCommon) dataUpdate(seq uint64, event sgbucket.FeedEvent) {
-	c.updateSeq(event.VbNo, seq, true)
 	shouldPersistCheckpoint := c.callback(event)
+	c.updateSeq(event.VbNo, seq, true)
 	if c.persistCheckpoints && shouldPersistCheckpoint {
 		c.incrementCheckpointCount(event.VbNo)
 	}