mysql store: several lint errors

[llorllale]

I've been forced to update golangci-lint while working on #75 (because http.Request.Header.Value does not exist in whatever go version is shipped with golangci-lint 1.21) and found these errors:

pkg/storage/mysql/mysqlstore.go:166:30: rows.Err must be checked (rowserrcheck)
        rows, err := newDBConn.Query(
                                    ^
pkg/storage/mysql/mysqlstore.go:318:31: rows.Err must be checked (rowserrcheck)
        resultRows, err := s.db.Query(findQuery)
                                     ^
pkg/storage/mysql/mysqlstore.go:388:30: rows.Err must be checked (rowserrcheck)
        rows, err := indexStmt.Query(s.tableName)
                                    ^
pkg/storage/mysql/mysqlstore.go:383:32: Rows/Stmt was not closed (sqlclosecheck)
        indexStmt, err := s.db.Prepare(getIndexStmt)

This task is a followup to fix these linting issues.