Fix "database table is locked" #9

This PR solves the problem that occures if you try to migrate a repo to a sqlite gitea instance. It just retries if a "database table is locked" occures.

Needs the approval of another contributor.

CC @techknowlogick @kolaente

It seems that this problem is a bug of gitea since the issue get's created but it fails while setting the status:

2018/06/18 09:04:29 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:29 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:30 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:31 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:36 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:39 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: comment
2018/06/18 09:04:40 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:42 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:43 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label
2018/06/18 09:04:44 [...api/v1/repo/issue.go:203 CreateIssue()] [E] ChangeStatus: database table is locked: label

I've created an issue at go-gitea/gitea: https://github.com/go-gitea/gitea/issues/4268

So does this mean you could resolve the issue on your side and everything is ok? Or do we need to wait for this to get resolved in Gitea?

@kolaente The problem is that if this problem occures no comments for the specific issue will be created. I thinknthat I will find a solution which does not depend on gitea.

I close this PR since this issue must be resolved at Gitea.