支持多线程同时读写 about bitcask-rs

[gfreezy]

struct ActiveData {
    segment: Segment,
    hashmap: HashMap<Key, Position>,
}

struct OlderData {
    segments: HashMap<u64, Segment>,
    hashmap: HashMap<Key, Position>,
}

pub struct Store {
    path: PathBuf,
    next_file_id: Arc<RwLock<u64>>,
    older_data: Arc<RwLock<OlderData>>,
    active_data: Arc<RwLock<ActiveData>>,
    pending_data: Arc<RwLock<OlderData>>,
}

merge ->

1. read_lock active_data pending_data older_data 获取需要 merge 的 file ids
2. read_lock older_data 把 file ids 做 merge 操作（费时间）
3. write_lock active_data pending_data try_write_lock older_data 把 merge 的结果回写

read -> read_lock active_data pending_data older_data
write -> write_lock active_data pending_data try_write_lock older_data