stopcursor

Implement `&stopcursor=<opaque string>` to act as an artificial terminus on a per-request basis.
Since you're now defining a properly bounded range, the following could make this signature and downstream related code more obvious.
```go
// Selecter defines the methods to retrieve elements from a sorted set.
type Selecter interface {
    // ...
    SelectCursor(keys []string, start, stop common.Cursor, limit int) <-chan Element
}
```