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 
nBlockSequenceId = 1;
    m_failed_blocks.clear();
    setBlockIndexCandidates.clear();
}
void UnloadBlockIndex()
{
    LOCK(cs_main);
    chainActive.SetTip(nullptr);
    pindexBestInvalid = nullptr;
    pindexBestHeader = nullptr;
    mempool.clear();
    mapBlocksUnlinked.clear();
    vinfoBlockFile.clear();
    nLastBlockFile = 0;
    setDirtyBlockIndex.clear();
    setDirtyFileInfo.clear();
    versionbitscache.Clear();
    for (int b = 0; b < VERSIONBITS_NUM_BITS; b++) {
        warningcache[b].clear();
    }
    for (const BlockMap::value_type& entry : mapBlockIndex) {
        delete entry.second;
    }
    mapBlockIndex.clear();
    fHavePruned = false;
    g_chainstate.UnloadBlockIndex();
}
bool LoadBlockIndex(const CChainParams& chainparams)