bench: add insert with prepare and select point/range test
bench: add insert with prepare and select point/range test

1. select range test may cost tons of time now, make N smaller (e.g.: 100)
2. modify cost time unit to second fixedly instead of %v

pingcap/tidb-bench,pingcap/tidb-bench,pingcap/tidb-bench,pingcap/tidb-bench,pingcap/tidb-bench,pingcap/tidb-bench,pingcap/tidb-bench