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

```
} 
    assert(stats);
    LOCK(m_cs_fee_estimator);
    
    if (confTarget <= 0 || (unsigned int)confTarget > stats->GetMaxConfirms())
        return CFeeRate(0);
    if (successThreshold > 1)
        return CFeeRate(0);
    double median = stats->EstimateMedianVal(confTarget, sufficientTxs, successThreshold, nBestSeenHeight, result);
    if (median < 0)
        return CFeeRate(0);
    return CFeeRate(llround(median));
}
unsigned int CBlockPolicyEstimator::HighestTargetTracked(FeeEstimateHorizon horizon) const
{
    LOCK(m_cs_fee_estimator);
    switch (horizon) {
    case FeeEstimateHorizon::SHORT_HALFLIFE: {
        return shortStats->GetMaxConfirms();
    }
    case FeeEstimateHorizon::MED_HALFLIFE: {
        return feeStats->GetMaxConfirms();
    }
    case FeeEstimateHorizon::LONG_HALFLIFE: {
        return longStats->GetMaxConfirms();
    }
    } 
    assert(false);
}
unsigned int CBlockPolicyEstimator::BlockSpan() const