Running on recent master, commit f1dbf92ff0475a01d20170ea422c1d086acbbc57

If I run:
```
test/functional/feature_fee_estimation.py --randomseed 108574360997204915
```

I get:
```
2020-12-19T20:14:06.175000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_bcwexp78
2020-12-19T20:14:07.738000Z TestFramework (INFO): This test is time consuming, please be patient
2020-12-19T20:14:07.738000Z TestFramework (INFO): Splitting inputs so we can generate tx's
2020-12-19T20:14:10.870000Z TestFramework (INFO): Finished splitting
2020-12-19T20:14:12.104000Z TestFramework (INFO): Will output estimates for 1/2/3/6/15/25 blocks
2020-12-19T20:14:12.104000Z TestFramework (INFO): Creating transactions and mining them with a block size that can't keep up
2020-12-19T20:14:15.879000Z TestFramework (INFO): Creating transactions and mining them at a block size that is just big enough
2020-12-19T20:14:21.104000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/sdaftuar/ccl-bitcoin/2020-12-latest-master/test/functional/test_framework/test_framework.py", line 126, in main
    self.run_test()
  File "test/functional/feature_fee_estimation.py", line 256, in run_test
    check_estimates(self.nodes[1], self.fees_per_kb)
  File "test/functional/feature_fee_estimation.py", line 140, in check_estimates
    check_smart_estimates(node, fees_seen)
  File "test/functional/feature_fee_estimation.py", line 130, in check_smart_estimates
    % (feerate, last_feerate))
AssertionError: Estimated fee (0.000565) larger than last fee (0.000479) for lower number of confirms

```
feeCalc->reason = FeeReason::DOUBLE_ESTIMATE;
        }
    }
    if (conservative || median == -1) {
        double consEst =  estimateConservativeFee(2 * confTarget, &tempResult);
        if (consEst > median) {
            median = consEst;
            if (feeCalc) {
                feeCalc->est = tempResult;
                feeCalc->reason = FeeReason::CONSERVATIVE;
            }
        }
    }
    if (median < 0) return CFeeRate(0); 
    return CFeeRate(llround(median));
}
void CBlockPolicyEstimator::Flush() {
    FlushUnconfirmed();
    FlushFeeEstimates();
}
void CBlockPolicyEstimator::FlushFeeEstimates()
{
    AutoFile est_file{fsbridge::fopen(m_estimation_filepath, "wb")};
    if (est_file.IsNull() || !Write(est_file)) {
        LogPrintf("Failed to write fee estimates to %s. Continue anyway.\n", fs::PathToString(m_estimation_filepath));
    } else {
        LogPrintf("Flushed fee estimates to %s.\n", fs::PathToString(m_estimation_filepath.filename()));
    }
}
bool CBlockPolicyEstimator::Write(AutoFile& fileout) const