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

```
{
    if (firstRecordedHeight == 0) return 0;
    assert(nBestSeenHeight >= firstRecordedHeight);
    return nBestSeenHeight - firstRecordedHeight;
}
unsigned int CBlockPolicyEstimator::HistoricalBlockSpan() const
{
    if (historicalFirst == 0) return 0;
    assert(historicalBest >= historicalFirst);
    if (nBestSeenHeight - historicalBest > OLDEST_ESTIMATE_HISTORY) return 0;
    return historicalBest - historicalFirst;
}
unsigned int CBlockPolicyEstimator::MaxUsableEstimate() const
{
    
    return std::min(longStats->GetMaxConfirms(), std::max(BlockSpan(), HistoricalBlockSpan()) / 2);
}
/** Return a fee estimate at the required successThreshold from the shortest
 * time horizon which tracks confirmations up to the desired target.  If
 * checkShorterHorizon is requested, also allow short time horizon estimates
 * for a lower target to reduce the given answer */
double CBlockPolicyEstimator::estimateCombinedFee(unsigned int confTarget, double successThreshold, bool checkShorterHorizon, EstimationResult *result) const
{
    double estimate = -1;
    if (confTarget >= 1 && confTarget <= longStats->GetMaxConfirms()) {
        
        if (confTarget <= shortStats->GetMaxConfirms()) { 
            estimate = shortStats->EstimateMedianVal(confTarget, SUFFICIENT_TXS_SHORT, successThreshold, nBestSeenHeight, result);
        }
        else if (confTarget <= feeStats->GetMaxConfirms()) {