m04：把配置参数收回去

struct Options {
    double compaction_ratio = 0.5;
    uint64_t compaction_min_bytes = 1 << 20;
    uint32_t compaction_check_interval = 100;
    size_t index_reserve = 1024;
    bool sync_writes = false;
    std::string validate() const;
};

void Database::maybeCompact() {
    if (++writesSinceCheck_ < options_.compaction_check_interval) return;
    writesSinceCheck_ = 0;
    uint64_t garbage = store_.dead();
    if (garbage < options_.compaction_min_bytes) return;
    if (static_cast<double>(garbage) <
        options_.compaction_ratio * static_cast<double>(store_.total())) return;
    store_.compact();
}

struct Options {
    // Call fsync(2) after every write command, so that an acknowledged write
    // survives a power failure. Without it, a write survives a crash of
    // minikv but may be lost if the machine goes down. ...
    bool sync_writes = false;
};

// Compaction rewrites only the live data, so it costs about `live` bytes of
// I/O and reclaims `garbage` bytes. Waiting until garbage >= live means every
// byte compaction writes pays for at least one byte of garbage removed: total
// disk writes stay within 2x of what the commands wrote, and the log stays
// within about 2x the size of the live data. ...
void Database::maybeCompact() {
    uint64_t garbage = store_.dead();
    uint64_t live = store_.total() - garbage;
    if (garbage >= kMinGarbageToCompact && garbage >= live) store_.compact();
}