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 
int64_t nMempoolUsage = mempool.DynamicMemoryUsage();
    LOCK(cs_main);
    static int64_t nLastWrite = 0;
    static int64_t nLastFlush = 0;
    std::set<int> setFilesToPrune;
    bool full_flush_completed = false;
    try {
    {
        bool fFlushForPrune = false;
        bool fDoFullFlush = false;
        LOCK(cs_LastBlockFile);
        if (fPruneMode && (fCheckForPruning || nManualPruneHeight > 0) && !fReindex) {
            if (nManualPruneHeight > 0) {
                FindFilesToPruneManual(setFilesToPrune, nManualPruneHeight);
            } else {