add report_bg_io_stats option

tiny cleanup

public get property function

*: support get property

save path for later use

*: add an option to disable write-ahead-log.

option: add max_bytes_for_level_base option

options: add compression option

export `iterator.valid()`

* seperate ffi crate
* fix clippy warning

Disable clippy

Clippy can't build with nightly rust.

support snapshot get/get_cf

wraper: add flush and approximate size support

*: clean up