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 
{
    LOCK(cs);
    std::vector<const CTxMemPoolEntry*> entries;
    for (const auto& tx : vtx)
    {
        uint256 hash = tx->GetHash();
        indexed_transaction_set::iterator i = mapTx.find(hash);
        if (i != mapTx.end())
            entries.push_back(&*i);
    }
    
    if (minerPolicyEstimator) {minerPolicyEstimator->processBlock(nBlockHeight, entries);}
    for (const auto& tx : vtx)
    {
        txiter it = mapTx.find(tx->GetHash());
        if (it != mapTx.end()) {
            setEntries stage;
            stage.insert(it);
            RemoveStaged(stage, true, MemPoolRemovalReason::BLOCK);
        }
        removeConflicts(*tx);
        ClearPrioritisation(tx->GetHash());
    }
    lastRollingFeeUpdate = GetTime();
    blockSinceLastRollingFeeBump = true;
}
void CTxMemPool::_clear()