add buffer to FEMU Blackbox

[AkmalArifin]

I implemented buffer mechanism in FEMU Blackbox, I also add interface configuration for setting its buffer size and buffer threshold

[wangyuyue]

Thank you for this PR. I notice that the LRU cache is currently only triggered by flash writes. Would adding missed read requests to the flash cache also enhance performance? If so, could you consider using dirty labels to differentiate between clean pages and those requiring write-back during eviction?

[AkmalArifin]

Sorry for the late response. After I adding missed read request to the flash cache, it actually increase the hit ratio constantly after several time of reading.

And for the using dirty labels, I'm sorry could you please elaborate more? Do i just need to add label for the pages that needs to be evicted.