To avoid deadlock problem, almost database related functions should be
have ctx as the first parameter.
This PR do a refactor for some of these functions.
This PR
- [x] Move some functions from `issues.go` to `issue_stats.go` and
`issue_label.go`
- [x] Remove duplicated issue options `UserIssueStatsOption` to keep
only one `IssuesOptions`
This PR
- [x] Move some code from `issue.go` to `issue_search.go` and
`issue_update.go`
- [x] Use `IssuesOptions` instead of `IssueStatsOptions` becuase they
are too similiar.
- [x] Rename some functions
