Polish search by skipping over as many candidate as possible

[wangkuiyi]

Now we have the following functions in search.go not yet implemented:

func (f *Frontier) findPivotTerm(threshold float64) int {
    // TODO(y): Implement this.
    return 0
}

// pickTerm returns a value in range [0, pivotTermIdx), or -1 for error.
func (f *Frontier) pickTerm(pivotTermIdx int) int {
    // TODO(y): Implement this.
    return -1
}