Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
{
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() {