cache file descriptor

[k-jingyang]

solves #9

[k-jingyang]

Although we mentioned using BufReader instead of File in #9, I think that File is just enough abstraction to hold the file descriptor. Caching BufReader limits how we want to use the file descriptor (i.e. maybe we use something else instead of BufReader next time).

[marvin-j97]

lsm-tree currently uses BufReader<File>, so it's not really compatible right now

Also when using a BufReader, we can still look inside it to get the File

[k-jingyang]

lsm-tree currently uses BufReader<File>, so it's not really compatible right now

Also when using a BufReader, we can still look inside it to get the File

Got it, let me check this out

[marvin-j97]

Probably should add a simple test for the FdCache (hit, no hit)

[k-jingyang]

Probably should add a simple test for the FdCache (hit, no hit)

Agree, I will follow up on this

[k-jingyang]

Probably should add a simple test for the FdCache (hit, no hit)

Agree, I will follow up on this

Added the test in basic_kv.rs. Do let me know your comments