When the mempool fills the verbose `getrawmempool` RPC call gets really slow.  It can take minutes for >100k transactions in the mempool.

The culprit is the univalue library.  Objects are implemented as a big list, but the library checks for duplicate keys.  So inserting n key-value pairs into an object takes O(n^2).

Possible fixes:

- Improve univalue library by using linked hash maps.
- change the RPC output format of getrawmempool to use a list of objects, instead of an object of objects.  But this breaks existing code.
- hack the library to be able to skip the duplicate check.

{
private:
    bool m_could_reserve;
public:
    explicit CoinsViewScanReserver() : m_could_reserve(false) {}
    bool reserve() {
        assert (!m_could_reserve);
        std::lock_guard<std::mutex> lock(g_utxosetscan);
        if (g_scan_in_progress) {
            return false;
        }
        g_scan_in_progress = true;
        m_could_reserve = true;
        return true;
    }
    ~CoinsViewScanReserver() {