m08：先测量，再改关键路径

std::string Server::handleGet() {
    ...
        if (!db_.exists(key)) return "(nil)";   // lookup #1, counts a hit
        auto r = db_.get(key);                  // lookup #2, counts again
        return bulk(r.second, true, true);      // what do true, true mean?
    ...
}

std::pair<bool, std::string> Store::get(const std::string& key) const {
    std::optional<std::string> v = index_.get(key);       // copy #1
    stats_.add(v ? "keyspace_hits" : "keyspace_misses");
    if (!v) return {false, ""};
    return {true, *v};                                      // copy #2
}

std::string Server::handleGet() {
    ...
        const std::string* value = db_.find(key);
        return value ? quoted(*value) : "(nil)";
    ...
}

// Returns a pointer to the value stored under `key`, or nullptr if there
// is none. The pointer is valid only until the next write to this Store
// (set, del, incr, append, compact); copy the value to keep it longer.
// Each call counts as one keyspace hit or miss in STATS.
const std::string* Store::find(const std::string& key) const;

// Whether `key` has a value. Unlike find(), not counted in STATS.
bool Store::exists(const std::string& key) const;

// Formats s for a reply.
std::string Server::bulk(const std::string& s, bool quote, bool escape) const {
    std::ostringstream out;
    if (quote) out << '"';
    for (char c : s) {
        unsigned char u = static_cast<unsigned char>(c);
        if (escape && (c == '"' || c == '\\')) {
            out << '\\' << c;
        } else if (escape && !std::isprint(u)) {
            out << "\\x" << std::hex << std::setw(2) ...;
        } else {
            out << c;
        }
    }
    ...
}
return bulk(db_.stats(), false, false);   // just a copy, in disguise

// True for bytes that cannot appear as-is inside a quoted reply. Printable
// ASCII is 0x20..0x7E; we test the range directly instead of calling
// std::isprint() per byte, which showed up in the GET benchmark.
bool needsEscape(unsigned char c);

// Appends `s` to `out` as a double-quoted reply string ... Runs of bytes
// that need no escaping (normally the whole value) are copied in one go.
void appendQuoted(std::string& out, const std::string& s);
std::string quoted(const std::string& s);

return db_.stats();