multithread on get_postlist

multimatch.cc

// It need push_back postlist by order, so record pl_map[i] = pl, then push them orderly.
std::map<int, PostList *> pl_map;
std::vector<std::thread> vt;
std::mutex mutex;
auto start = std::chrono::high_resolution_clock::now();
for (size_t i = 0; i != leaves.size(); ++i) {
    vt.push_back(std::thread([this, first, maxitems, start, &total_subqs, &pl_map, &definite_matches_not_seen, &mutex](size_t ii) {
        PostList *pl = leaves[ii]->get_postlist(this, &total_subqs);
        if (false && is_remote[ii]) { // use localmutltimatch, so don't go here
            if (pl->get_termfreq_min() > first + maxitems) {
                LOGLINE(MATCH, "Found " <<
                                        pl->get_termfreq_min() - (first + maxitems)
                                        << " definite matches in remote submatch "
                                           "which aren't passed to local match");
                definite_matches_not_seen += pl->get_termfreq_min();
                definite_matches_not_seen -= first + maxitems;
            }
        }
        {
            // map should be thread safe
            std::lock_guard<std::mutex> guard(mutex);
            pl_map[ii] = pl;
        }
    }, i));
}
std::for_each(vt.begin(), vt.end(), [](std::thread &t) { t.join(); });
for (size_t i = 0; i != leaves.size(); ++i) {
    postlists.push_back(pl_map[i]);
}