[writeboost] Add benchmark to how writeback optimization effects

Add new benchmarking to see how writeback optimization effects.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>

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This test is to measure how effective the writeback optimization of Writeboost is by comparing the time to carry all write dirty data to HDD.

w/ Writeboost: write on SSD -> wait for all dirty data are written back to HDD
wo : Just writes to HDD.

The result in my environment is

```
Elapsed 67.258316296: writeboost batch_size(4)
Elapsed 45.095309196: writeboost batch_size(32)
Elapsed 35.220709076: writeboost batch_size(128)
Elapsed 85.789129318: backing ONLY
```

This result indicates stacking Writeboost on a HDD can boost writes.

(Oh, ... why it deteriorates reads this much)